Genetics of Nickel Allergic Contact Dermatitis
Sarah E. Schram; Erin M. Warshaw Dermatitis. 2007;18(3):125-133. ?2007 American Contact Dermatitis Society
Abstract and Introduction
Nickel sulfate is the most frequently detected cause of allergic contact dermatitis in the world; the prevalence of nickel allergic contact dermatitis is between 8 and 11% in the general female population. Although it is well recognized that environmental factors are important in the pathogenesis of this dermatitis, some investigators have hypothesized that genetic factors are important as well. This review summarizes animal and human studies evaluating genetic factors in the development of allergic contact dermatitis from nickel.
NICKEL is ubiquitous in our society as it is present in common objects such as jewelry, coins, spectacle frames, and zippers. Occupational sources of nickel exposure include nickel plating, alkaline batteries, insecticides, fuel additives, dyes, and pigments. Allergic contact dermatitis (ACD) from nickel was first recognized in the early 1930s and continues to be a significant problem; prevalence rates in the general population are 8 to 11% in women and 1 to 2% in males.[2,3] Among patients patch-tested for suspected ACD, approximately 14 to 20% have positive reactions to nickel sulfate.[4-8] On the basis of large aggregated databases from patch-testing centers, nickel sulfate has consistently been demonstrated to be the most frequently detected allergen worldwide. Risk factors for the development of nickel ACD include female sex, young age, and a history of ear piercing.[9,10] Although the economic impact of nickel ACD has not been extensively studied, it is known that nickel ACD can result in occupational disability. In a group of 564 Danes applying for permanent disability pensions because of skin diseases, 17.5% were found to have nickel allergy.
Environmental factors are undoubtedly of considerable importance in the development of nickel ACD. Several researchers have hypothesized that genetics may also play a role in susceptibility to and development of nickel hypersensitivity. Genetic predispositions have been found in other allergic diseases (such as dermatitis herpetiformis and gluten sensitivity), in fixed drug eruptions, and in allergies to specific drugs such as pyrazolone.[12-14] To the best of our knowledge, contact dermatitis from topically applied allergens has never been conclusively linked to genetic factors in humans. The purpose of this review is to summarize animal and human studies evaluating genetic factors in the development of ACD from nickel.
A PubMed search was conducted with the search terms "nickel allergy AND genetics," "nickel allergic contact dermatitis AND genetics," and "allergic contact dermatitis AND genetics." A manual search was also conducted to obtain articles whose publication dates were not included in PubMed.
When published studies did not report statistical analyses, we performed these analyses with SAS software (Statistical Analysis System, SAS Institute Inc., Cary, NC) and identified them as "calculated" in the text. Calculations of associations were completed with chi-square tests. A significance level of .05 was used in all analyses.
Early evidence for the inheritability of sensitization to allergens came from animal studies. Animal models for nickel hypersensitivity were initially difficult to obtain owing to problems with sensitization. Nickel can be toxic in small doses and is difficult to dissolve in an organic medium. Therefore, many early studies did not examine nickel but instead investigated the inheritability of allergy to other simple compounds.
In 1941, Chase bred guinea pigs that were selected for high and low susceptibility to sensitization with 2,4-dinitrochlorobenzene (DNCB) and studied susceptibility in their progeny. Animals were sensitized with 2.5 ?g of DNCB in 0.1 cc saline injections over a 6-week period. Susceptibility was tested by spreading a drop of a 1% solution of DNCB in olive oil on the belly. Reactivity was examined the next day by an observer who was blinded to the origin of the animals. Reactvity was rated with the following scale: I (pink, often slightly elevated), II (pale pink), III (faint pink), and IV (negative or minimal reaction). High-susceptibility animals (grades I, I/II, and II) were bred with other high-susceptibility animals and vice versa, to select animals appropriate for starting colonies, with a result of 112 high-susceptibility descendants and 110 low-susceptibility descendants. The investigators then tested the two cohorts after brief and long sensitization periods. The high-susceptibility animals tended to have high-susceptibility progeny: 88 of 111 animals (79%) were graded between I+ and II after brief sensitization, and 62 of 67 animals (93%) were so graded after long sensitization. Low-susceptibility animals tended to have low-susceptibility progeny: only 10 of 105 animals (10%) were graded between I+ and II after brief sensitization, and 36 of 93 animals (39%) were so graded after a longer course of sensitization. The difference in proportions of high-susceptibility progeny from high-susceptibility parents and from low-susceptibility parents was statistically significant (calculated p < .0001) after both brief and long sensitization procedures. The progeny of high-susceptibility animals were 8.3 times more likely to react strongly after brief sensitization than those of low-susceptibility animals (calculated 95% confidence interval [CI], 4.58-15.13). The relative risk of the progeny of high-susceptibility animals was 2.39 (calculated 95% CI, 1.83-3.11) after long sensitization, compared to low-susceptibility animals. Chase concluded that there was a genetic basis for susceptibility to contact allergy to DNCB in the guinea pig. However, limitations of this study include subjectivity of grading, lack of high versus low crossbreeding, and exclusion of some breeders by an unclear "progeny test."
In 1963, Levine and colleagues used guinea pigs to study the genetic transmission of the capacity to become immunized by hapten-poly-L-lysine (hapten-PLL) conjugates, antigens of relative structural simplicity. Sixty-seven randomly bred Hartley guinea pigs were immunized with either benzylpenicilloyl PLL (BPO-PLL) or 2,4-dinitrophenyl PLL (DNP-PLL). The offspring of responders and the offspring of nonresponders were immunized with DNP-PLL; 82% of 18 progeny from 8 breeding pairs of responder parents showed an immune response to intradermal DNP-PLL (10 ?g in 0.1 mL of saline), with delayed allergic skin reactions. None of the 26 offspring of 9 breeding pairs of nonresponder parents showed a demonstrable immune response. The authors also immunized two other strains (II and XIII) of guinea pigs with DNP-PLL and BPO-PLL. All 40 guinea pigs immunized with strain II developed immune responses to these conjugates whereas none of the 11 guinea pigs immunized with strain XIII demonstrated an immune response. The authors concluded that the ability to respond immunologically to hapten-PLL conjugates is transmitted genetically as an autosomal mendelian dominant trait.
In 1968, Polak and colleagues evaluated the differential ability of the same two inbred strains (II and XIII) of guinea pigs used in Levine and colleagues' study, as well as outbred Hartley guinea pigs, to develop ACD from inorganic metal compounds. The metal compounds they used included potassium dichromate (K2Cr2O7), beryllium fluoride (BeF2), and mercuric chloride (HgCl2). They also investigated the inheritance of reactivity to BeF2 by mating Hartley guinea pigs that had different degrees of sensitivity. Sensitization and skin-testing techniques differed for each compound. All metals were tested by skin painting; the reactions were read at 24 hours and were graded on a three-point scale. Sixty-six percent of 73 Hartley guinea pigs, 80% of 10 strain II guinea pigs, and none of 8 strain XIII guinea pigs could to be sensitized to K2Cr2O7; 79% of 33 Hartley guinea pigs, 72% of 11 strain II guinea pigs, and none of 10 strain XIII guinea pigs could be sensitized to BeF2; and 46% of 15 Hartley guinea pigs, none of 8 strain II guinea pigs, and 80% of 10 strain XIII animals could be sensitized to HgCl2. In the experiments of inheritance of BeF2 reactivity, two strong-reacting Hartley guinea pigs were mated; all (100%) of the three progeny were strong reactors. From three breeding pairs in which one parent was a strong reactor and the other was a nonreactor, 64% of 11 progeny were strong reactors, 18% were mild reactors, and 18% were nonreactors. In two breeding pairs of mild reactors and nonreactors, 43% of 7 progeny were strong reactors, 14% were mild reactors, and 43% were nonreactors. The authors concluded that in the Hartley guinea pig, the ability to react to BeF2 is inherited as a simple mendelian dominant characteristic and that the ability to react to different metals is controlled independently.
In a 1990 study, Ishii and colleagues successfully sensitized H-2 congenic and recombinant strains of mice to nickel. H-2 is the mouse major histocompatibility (MHC) antigen and is divided into classes I and II. In humans, the MHC antigen is human leukocyte antigen (HLA). The mouse class II complex is known as the I region and is divided into A and E loci. Congenic and recombinant mice were sensitized to nickel and then challenged with 25 ?L of 0.4% nickel sulfate in physiologic saline injected into both hind footpads. The response was calculated as the difference (in 10-2 millimeters) of thickness before and 24 hours after challenge. Ten strains of mice showed high responses to nickel sulfate challenge, and four showed low responses. Recombinant studies demonstrated that the I-A subregion of mouse H-2 was important for these responses.
Although many of these animal studies did not specifically investigate nickel, these reports indicate the importance of genetic factors and a possible role for MHC molecules in the development of ACD from nickel and other compounds.
Early human studies of the genetics of ACD included sensitivity to several antigens. In the 1960s, Forsbeck and colleagues conducted an uncontrolled pilot study evaluating relatives of patients with ACD from a variety of antigens. Relatives of probands (including parents, siblings, and children older than 10 years) were interviewed with regard to symptoms of ACD, examined, and patch-tested with a panel of 23 common allergens. Of the siblings and children of probands, 23 of 93 (25%) had positive patch-test results (data for nickel were not reported separately). Although they did not have a control group for comparison, the authors concluded that these preliminary results suggested an increased incidence of positive patch-test reactions in the children and siblings of patients with ACD.
Subsequently, Forsbeck and colleagues compared the prevalence of ACD in first-degree relatives of patients with ACD to the prevalence of ACD in relatives of controls. The probands were consecutive patients with ACD who were admitted to a skin clinic in Stockholm. The diagnosis was based on history and patch testing, and 94 probands (48 women and 46 men) ranging in age from 16 to 79 years were enrolled in the study. Of the probands, 19 of 94 (20%) had nickel ACD. The patient's parents, siblings, and children older than 10 years were considered first-degree relatives. Relatives were questioned regarding the history of ACD or current ACD, examined for the presence of ACD, and patch-tested. The control group included the wives and husbands of the probands and a group of selected twins, one from each pair, who had been used as subjects in a previous investigation (matched status not reported). Controls and their relatives were investigated in the same way as were probands and their relatives. Not all first-degree relatives agreed to participate; 22% of relatives of probands and 35% of relatives of control patients refused testing. Patch testing included a series of 23 common allergens which included 5% nickel sulfate in water. Overall, 101 (25%) of 404 relatives of probands had positive patch-test results, and 27 (17.3%) of 156 relatives of controls had positive patch-test results. This difference approached statistical significance (calculated p = .052; relative risk [RR], 1.44; 95% CI, 0.99-2.12). These results were further analyzed by sex; 62 (29.8%) of 208 relatives of female probands had positive patch-test results whereas 19 (18.3%) of 104 relatives of female controls had positive patch-test results. This difference was significant (p < .05). Relatives of female probands were 1.63 times more likely to develop ACD than relatives of female controls (calculated 95% CI, 1.03-2.58). Thirty-nine (19.9%) of 196 relatives of male probands had positive patch-test reactions, and 8 (15.4%) of 52 relatives of male controls also had positive results (calculated p > .05). The authors concluded that genetic constitution may be important for the development of ACD, but this study examined ACD in general and did not report results for nickel separately. Other limitations included unmatched controls and the large percentage of relatives who refused testing.
Fleming and colleagues investigated familial disposition to nickel ACD in 258 patients prospectively evaluated for routine patch testing. Thirty-nine patients with an unequivocal history of nickel ACD and positive patch-test results were considered probands; 84 patients with no history of nickel ACD and negative patch-test results for nickel were used as controls. (The number of controls who had ACD from other antigens was not reported.) Patients were required to contact all first-degree relatives and discuss a questionnaire seeking information about nickel intolerance. Thirty-one (15%) of 209 first-degree relatives of nickel-allergic probands reported a history suggestive of nickel ACD as compared with 24 (5.2%) of 458 first-degree relatives of controls who had a history suggestive of nickel ACD (RR, 2.83; 95% CI, 2.45-3.27). The authors concluded that these data contribute to evidence suggesting that genetic elements are important in nickel ACD. Possible confounders included ear piercing, other nickel exposures, and gender, none of which were controlled for. Also, questionnaire administration may not have been uniform because the interviews of first-degree relatives were conducted by the subjects instead of trained investigators. Biases also include possible increased recognition of nickel ACD in relatives of probands or increased exposure to nickel in families (for example, from a tendency toward ear piercing).
Several researchers evaluated twins for evidence of genetic predisposition for nickel hypersensitivity. Menne and Holm compared the concordance of nickel ACD between Danish female monozygotic (MZ) twins and dizygotic (DZ) twins. Their sample included female twins in the Danish Twin Register who were born between 1906 and 1930 and who were alive and available in 1978; 1,546 pairs were mailed a questionnaire about skin reactions to metal directly in contact with the skin, such as jewelry. Investigators attempted to interview those twins who lived in the eastern portion of the country. Twins were considered to have ACD by history if they reported at least two episodes of itching and rash related to metal objects worn next to the skin. Of the 1,492 individuals in the eastern portion of the country, 129 individuals in 115 pairs gave an answer other than "no" to the question regarding metal sensitivity. Of these, 86 pairs were interviewed in person. Information was obtained from 14 other pairs by (1) personal visitation with one sister, (2) a twin giving information about a co-twin, (3) telephone interview, or (4) letter. Incomplete information was obtained for another eight pairs, and seven pairs were interviewed only by telephone. Of the 230 individuals in 115 pairs, 105 individuals had a positive history of nickel allergy (as determined by interview), 100 had a negative history, and 25 had incomplete information. These data were then compared by zygosity. By interview, the pairwise concordance rate for MZ twins was 0.32 (11 of 34); for DZ twins, it was 0.14 (7 of 50). This association was statistically significant (p < .05). Of the 86 pairs visited, 75 were also patch-tested. By patch testing, the pairwise concordance rate for MZ twins was 0.29 (6 of 21) as compared to 0.08 (2 of 26) for DZ twins (p > .05). The authors concluded that genetic factors may be of importance in the development of nickel ACD although their sample size was limited.
Forsbeck and colleagues investigated the concordance of patch-test results between MZ and DZ twins for common allergens. The study included 101 pairs (51 MZ and 50 DZ) of twins from the Swedish Twin Register who were born between 1886 and 1925. At the time of the study, the participants ranged in age from 40 to 80 years. Ear piercing status was not reported. Of the 51 MZ pairs, 36 were female; of the 50 DZ pairs, 32 were female-female. The twins were patch-tested with 23 common allergens, including 5% nickel sulfate in water. Two sets of MZ female twin pairs both had positive patch-test results, accounting for 4% of total MZ twin pairs and 6% of female MZ twin pairs. No male MZ pairs were concordant. One set of DZ female twins both had positive patch-test results, accounting for 2% of total DZ twin pairs and 3% of female DZ twin pairs. No DZ males were concordant. The difference in concordance rates between MZ and DZ female twins was not statistically significant. Twelve of 202 individuals had positive patch-test reactions to nickel; however, these data were not reported separately for concordance. These authors concluded that there was no evidence that genetic background is of importance in ACD.
Bryld and colleagues recruited a sample of Danish twins with hand eczema for evaluation of genetic factors in the development of nickel sensitization. The investigators sent questionnaires to 6,666 twin individuals. On the basis of positive answers regarding the presence of hand eczema, twin individuals were invited for a clinical examination and patch testing for nickel allergy. Between 1997 and 1998, 1,076 individual twins (697 females and 379 males) were patch-tested. In the final genetic analysis, 630 females were included. Thirty-seven individuals were excluded because examination of their co-twin was not possible, 24 were excluded for uncertain zygosity, 2 were part of triplets, and 2 did not fulfill proband criteria. Data from males were not analyzed because of low nickel allergy rates. Odds ratios were modeled with logistic regression of a twin's nickel allergy status conditioned on the co-twin's nickel allergy status. One hundred four pairs with two probands were included twice. The data were modeled twice to adjust for different risk factors associated with nickel ACD. In model 1, data were adjusted for age and zygosity; model 2 data were adjusted for age, zygosity, hand eczema, "wet work," and atopic dermatitis. In both models, there was a small but not statistically significant tendency for larger odds ratios (ORs) among MZ twins than among DZ twins (model 1: OR = 1.2, 95% CI, 0.34, 4.31; model 2: OR = 1.3, 95% CI, 0.33-5.00). The authors concluded that genetic factors play a lesser role than environment in the development of ACD. The limitations of this study were discussed in an article published in the same journal issue by Bataille and included the use of the variable phenotype hand eczema as a selection criterion and a relatively small sample size per age group.
Human Leukocyte Antigen Studies
ACD has been traditionally described as a delayed-type hypersensitivity reaction, and the interaction between Langerhans' cells (LCs) and T cells is critical for pathogenesis. This interaction is facilitated by antigen processing and presentation on HLA molecules by LCs and recognition by appropriate T-cell receptors. Recent research has indicated that nickel acts as an atypical hapten, activating T cells through a variety of mechanisms, including processes that may be HLA independent. Many different cells and cytokines are involved in the pathogenesis of ACD. A full discussion of these mechanisms is beyond the scope of this review; interested readers may find reviews by Li and Cruz, Kimber and Dearman, and Thierse and colleagues, helpful.
HLA molecules are encoded on chromosome 6 and are divided into class I, II, and III MHC molecules (Figure 1). Class I includes HLA-A, HLA-B, and HLA-C. In general, class I HLA molecules present endogenous antigens to CD8+ T cells. Class II includes HLA-DR, HLA-DQ, and HLA-DP. In general, class II HLA molecules present exogenous antigens to CD4+ T cells. Class III gene products include tumor necrosis factor and heat shock proteins. Variations in amino acids in the peptide-binding region of HLA molecules affect antigen binding specificity and may alter disease resistance or susceptibility. Dermatologic diseases that have been associated with HLA polymorphisms include pemphigus vulgaris and psoriasis.[29,31,32]
Nickel interaction with HLA antigens on the surface of antigen-presenting cells is one mechanism of T-cell activation. A number of studies comparing the frequency of HLA antigens in patients with nickel ACD to that in the general population have been performed ( Table 1 ). Some of these studies used the "etiologic factor" (also called "etiologic fraction") statistic. Etiologic factor (EF) is a statistic used to help determine how much of a pathogenetic mechanism is located in, or associated with, a certain gene. It is calculated by applying the following formula:
In this formula, "RR" is relative risk, "a" is the number of marker-positive patients, and "b" is the number of marker-negative patients. The EF ranges from 0 to 1; a value of 1 implies that 100% of a disease is caused by the studied factor.
Walton and colleagues studied the distribution of 25 HLA-A, -B, and 9 HLA-DR antigens in 26 English female patients who had positive patch-test reactions to nickel. Controls were obtained from the Yorkshire Regional Transfusion Center at Leeds (number of controls and matched status were not reported). The HLA-B35 antigen frequency was significantly increased in patients with nickel ACD as compared to the Yorkshire controls (p < .01, corrected for the number of antigens studied; RR, 2.82).
Mozzanica and colleagues examined the frequency of 51 HLA-A, -B, -C, and -DR antigens in 54 Italian patients who had unequivocal positive patch-test reactions to nickel. A group of 320 healthy blood donors from the same geographic area were used as controls. The study group consisted of 50 females and 4 males and ranged in age from 14 to 55 years. The HLA-DRw6 antigen was present in 20 (calculated) (37.7%) of the 54 patients as compared to 36 (calculated) of 230 (15.6%) of the controls (p < .0005 before correction and p < .025 after correction; RR, 3.32).
Onder and colleagues examined the frequency of 12 HLA-DR, -DQA, -DQB, and -DP antigens in 32 Turkish patients who had positive patch-test reactions to nickel. Subjects included 28 females and 4 males and ranged in age from 19 to 44 years; 25 persons of the same age and ethnicity were used as controls. HLA-DQA1*0601 was present in 13 (40.6%) of the 32 nickel-allergic patients and in 3 (12.0%) of the 25 controls (corrected p < .037; calculated RR, 1.75; 95% CI, 1.17-2.63). HLA-DR15 was less prevalent among nickel-allergic patients. It was present in 9 (28.1%) of the 32 nickel-allergic patients as compared to 15 (60%) of the 25 controls (corrected p < .032; calculated RR, 0.54; 95% CI, 0.31-0.95).
Olerup and Emtestam examined the distribution of 28 HLA-DRB, -DQA, and -DQB polymorphisms in 33 patients with strong positive patch-test reactions to nickel; 100 unrelated Swedes were used as controls. Twenty-two (67%) of the 33 patients with nickel ACD had a 4.5 kb DQA restriction fragment as compared to 31 (31%) of the 100 healthy controls. This difference was statistically significant (corrected p < .05; RR, 4.5; EF, 0.52). This study also investigated whether the magnitude of the response in a lymphocyte proliferative assay to nickel sulfate was under the control of HLA class II genes. In the lymphocyte proliferative assay, there were no significant differences in the frequencies of any of the HLA class II genes (DRB, DQA, and DQB) found between low, intermediate, and high responders.
Based on the findings of Olerup and Emtestam, Ikaheimo and colleagues investigated the frequency of the HLA-DQA TaqI restriction fragment length polymorphism (RFLP). The subjects included 96 Finnish patients who had positive patch-tests reactions to nickel. Controls included 92 blood donors, medical students, and laboratory personnel (matched status not reported). Ten DQA1 and 17 DQB1 alleles were determined. There were no statistically significant associations between the frequency of HLA-DQA1 or -DQB1 and ACD from nickel. Thus, the results of Olerup and Emtestam were not confirmed.
Emtestam and colleagues subsequently evaluated the frequency of HLA DQB, DRB, DPA, and DPB alleles in 37 patients who had positive patch-test reactions to nickel in order to confirm their previous finding of an association with an HLA-DQA1 RFLP. One hundred fifty randomly selected healthy Swedes were used as controls (matched status not reported). Data from this study were analyzed separately and then pooled with data from the 1988 publication. The 4.5 kb DQA restriction fragment, corresponding to the DQA*0501 allele, was present in 30% of nickel-allergic patients and in 41% of controls. After data from the two studies were pooled, the frequency was 42% in 70 nickel-allergic patients and 38% among 250 controls. These differences were not statistically significant. The authors concluded that there were no associations with the 4.5 kb DQA RFLP. They also noted that they could not confirm the association of HLA-Drw6 and nickel sensitivity reported by Mozzanica and colleagues. In Emtestam and colleagues' group of patients, the frequency of HLA-DRw6 was 31% as compared to 37% for controls.
Liden and colleagues examined the frequency of 21 HLA A and B alleles in 47 patients with strong positive patch-test reactions to nickel; 339 healthy blood donors from the same geographic region served as controls. Although initial analyses indicated an association of HLA-B8 with nickel ACD, this was not statistically significant after correction for the number of antigens studied. Subsequently, Liden and colleagues conducted another study of 21 HLA-A and -B antigens in patients with ACD. Fifty-six Swedish patients with positive patch-test results for nickel were used as subjects. These were matched with controls for age (plus or minus 5 years), sex, and place of residence. Again, no significant associations were found.
Kapoor-Pillarisetti and colleagues investigated the frequency of 28 HLA-A and -B antigens among 39 females (ethnicity not reported) who had positive patch-test reactions to nickel. The control population consisted of 648 unmatched Caucasian patients who had been tissue-typed at St. Mary's Hospital in London. Eleven (28.2% calculated) of the 39 subjects had the HLA antigen B21 as compared to 32 (4.9%) of the 648 controls. Before correction, this association was significant (p < .01), but after correction for the number of antigens studied, no statistically significant association was found.
Hansen and colleagues examined the frequency of 31 HLA-A and -B antigens in 27 pairs of female MZ twins and 34 pairs of female DZ twins in which at least one twin had a positive patch-test result for nickel; in 8 of the MZ pairs and 6 of the DZ pairs, both twins were affected. They also investigated HLA frequency in 51 unrelated females who had positive patch-test reactions to nickel. HLA frequencies among 5,202 unrelated and unmatched Danes were used as controls. HLA-A9, -B15, -Bw39, and -Bw45 were more common among subjects with nickel allergy; however, this association was not statistically significant after correction for the number of antigens studied.
Karvonen and colleagues determined 28 HLA-A, -B, and -C antigens in 38 patients and 13 HLA-DR and -MT antigens in 30 patients who had positive patch-test results for nickel. There were 283 controls for HLA-A and -B antigens, 138 for HLA-Cw1, -Cw2, -Cw3, and -Cw4, 37 for HLA-Cw7, and 58 for HLA-DR and -MT. All patients were from the same geographic region. Although initially HLA-B12 was found more frequently and HLA-DR1 and -MT1 were found less frequently in patients with nickel ACD, these associations were not statistically significant after p value correction.
In 1979, Silvennoinen-Kassinen and colleagues examined the frequency of 36 HLA A, B, C, and Dw alleles in a Finnish population of 36 subjects (35 females and 1 male) who had positive patch-test reactions to nickel as compared to 217 blood donors from the same geographic area. Of the 36 subjects, 9 were atopic. HLA-B8 was statistically increased in the atopic subgroup although this association was not statistically significant after correction for the number of antigens studied.
Silvennoinen-Kassinen and colleagues examined the HLA types of the families of 10 nickel-allergic probands to investigate whether a disease susceptibility gene controlling nickel allergy was inherited with an HLA haplotype. Two families were also HLA-typed because lymphocytes of one member of each family demonstrated a high reaction to nickel sulfate in vitro although the patient showed no clinical signs of nickel allergy. For controls, 1,175 haplotypes from normal Finnish persons were used. Nickel ACD was not associated with a specific HLA haplotype in patients. Additionally, none of the haplotypes were increased in the parental generation.
Dumont-Fruytier and colleagues studied the frequency of 29 specificities of HLA-A and -B antigens in 60 unrelated Caucasian females ranging in age from 13 to 67 years who had positive patch-test results for nickel; 747 healthy blood donors served as controls. No significant associations between patch-test status and HLA type were found.
Braathen and colleagues examined HLA-A, -B, -C, and -DR antigens in 53 patients who had positive patch-test reactions to nickel. HLA frequencies were determined serologically and compared with those of healthy Norwegians (number of controls and matched status were not reported). No significant associations with any HLA antigens were found.
Of the fifteen studies reviewed, only four found statistically significant associations after correction for the number of HLA antigens studied (see Table 1 ). These associations included an increased prevalence of HLA-B35, -DRw6, -DQA1*0601, and the TaqI 4.5 kb DQA RFLP and a decreased prevalence of HLA-DR15. The associations between HLA-DRw6 and the TaqI 4.5 kb DQA RFLP were not confirmed in later studies. The difficulty in elucidating consistent HLA associations is not limited to nickel ACD; similar inconsistent results have been obtained for chromium ACD.[39-41]
Studies Evaluating TAP Genes
After inconsistent results concerning HLA associations, investigators began looking for other disease susceptibility genes. Silvennoinen-Kassinen and colleagues evaluated whether TAP1 (transporter associated with antigen processing) and TAP2 genes were involved in susceptibility to nickel allergy. The products of TAP1 and TAP2 are involved in antigen transport, and it has been hypothesized that they may contribute to disease susceptibility through antigenic selection. Significant associations between TAP genes and diseases such as sarcoidosis and psoriasis vulgaris have been reported.[48,49] In one study, 55 nickel-sensitive patients were enrolled and 54 controls were randomly selected from unrelated residents of northern Finland. TAP1 and TAP2 dimorphisms were detected from genomic deoxyribonucleic acid (DNA). TAP1 amino acid positions 333 and 637 and TAP2 amino acid positions 379, 565 and 665 were examined. The allele frequency of TAP2B was 40% in nickel-sensitive patients and 26% in controls (p = .019; RR, 2.7; EF, 0.46). The phenotype frequency was 73% in nickel-sensitive patients and 50% in controls (p = .012). The allele frequency of TAP2C was 1% in patients and 7% in controls (p = .016; RR, 0.18); however, the difference in phenotypic frequency was not significant. The authors concluded that TAP2B increases the risk for nickel sensitivity.
Studies Evaluating T-Cell Receptors
Silvennoinen-Kassinen and colleagues compared the usage of T-cell receptor (TCR) ß gene variables in nickel-induced CD4+ and CD8+ cells of nickel-allergic versus nonallergic patients. Four adults who had positive skin test results for nickel were used as subjects, and three nonsensitized patients were used as controls. Heparinized venous blood was collected for in vitro testing. TCR Vß gene segments were determined before and after nickel stimulation through complementary DNA polymerase chain reaction (PCR) amplification and separation by agarose gel electrophoresis. The use of the Vß gene was considered elevated or decreased if the change expressed after nickel stimulation was at least 10% of the unstimulated value. In the CD4+ samples, nickel-sensitive subjects had increased usage of 2 to 4 Vß genes each (Vß10 and Vß13 in subject A; Vß1, Vß2, Vß13, and Vß21 in subject B; Vß1 and Vß10 in subject C; and Vß9 and Vß19 in subject D). Two nonsensitized subjects showed increased usage of Vß genes after nickel stimulation (Vß1, Vß9, and Vß10 in subject G; Vß7 and Vß9 in subject H). A single Vß gene did not dominate in CD4+ samples. In the CD8+ samples, all nickel-sensitized subjects had an increase in 1 to 2 Vß genes (Vß1 in subject A, Vß1 in subject B, Vß1 and Vß2 in subject C, and Vß7 in subject D). Two nonsensitized subjects showed increased usage of Vß genes in CD8+ cells after stimulation (Vß1 in subject E; Vß3 and Vß7 in subject H). Among nickel-sensitive subjects, the Vß1 gene dominated in most (75%) of the CD8+ samples. This preliminary study suggested that the pattern of Vß genes induced by nickel stimulation is individual.
Studies evaluating the role of genetics in ACD from nickel have yielded conflicting results and are limited by small sample size, poor matching techniques, selection bias, and open study designs. Early animal and family studies on the heritability of ACD indicated that genetic factors may play a role in the development of nickel ACD. The results of twin studies have been conflicting and many were uncontrolled for environmental exposures to nickel.
Investigations of specific genetic factors have been inconclusive. Nickel appears to act as an atypical hapten and activates T cells through a variety of mechanisms, which may make defining a specific genetic factor challenging.
The issue of variations in environmental exposure is critical. Without controlling for environmental exposure, it may be impossible to determine if there is a genetic predisposition to nickel ACD. In the case of twin studies, if one twin had significantly more nickel exposure than the other and developed nickel ACD, it would skew the data away from concordance even though the co-twin might also have developed nickel ACD with adequate exposure. It is also possible that those subjects labeled as nonallergic for the HLA, TAP, and TCR studies could develop nickel ACD after significant nickel exposure. In summary, more studies are needed to determine whether genetics play an important role in the development of nickel ACD. Although some studies control for ear piercing status, nickel exposure occurs in many other forms, including dietary ingestion and industrial exposure. Exposure to nickel is a major confounder and significant barrier to the understanding of the genetics of nickel ACD.
Table 1. Studies Evaluating the Association of Nickel Allergy and Human Leukocyte Antigen Type
Sarah E. Schram, Erin M. Warshaw, University of Minnesota and Veterans Affairs Medical Center, Minneapolis, MN