INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Diffuse panbronchiolitis (DPB) is a chronic obstructive pulmonary disease of unknown etiology (1, 2). It is characterized clinically by chronic cough, sputum, and dyspnea, physiologically by chronic airflow limitation, radiologically by hyperinflation and diffuse small nodules, and histologically by chronic inflammatory lesions around respiratory bronchioles. Although prevalent in Japan, it is known to be rare in other countries, and only a few cases have been reported among Chinese, Italians, and North Americans (3). However, the disease is not rare in Koreans, and since the initial report in 1992 (8), about 50 cases have been reported.

It has been suggested that environmental and genetic factors might be important in the pathogenesis of this disease. In this context, significantly increased frequency of human leukocyte antigen (HLA)-B54 was found in Japanese patients with DPB (9), and this finding has been reconfirmed in a recent study with a larger number of patients (10). It is of interest that the disease is associated with HLA-B54, an antigen known to be unique to East Asian ethnic groups (11). However, HLA association with DPB has not been studied in East Asian ethnic groups other than Japanese. The purpose of this study was to evaluate the possible association of HLA with disease susceptibility in Korean patients with DPB.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Patients

Blood samples were obtained from 30 Korean patients with DPB diagnosed in Seoul National University Hospital and Asan Medical Center. Twenty male and 10 female patients were studied, and the mean age at the time of diagnosis was 52 yr (range, 23-76 yr). Clinical diagnosis was made according to the clinical, functional, and radiologic criteria of Homma and colleagues (1) and the characteristic features of high-resolution computed tomography of the chest.

HLA Typing

HLA typing was performed in 30 patients for HLA-A, -B, and -C and in 20 patients for HLA-DR loci. HLA-A, -B, and -C serologic typing was done using a commercial kit designed for Asian populations (OT-72; One Lambda, Canoga Park, CA). HLA-DR low resolution DNA typing was performed using a commercial kit of reverse sequence-specific oligonucleotide hybridization method (Amplicor HLA DRB Test; Roche, Branchburg, NJ). Frequencies of HLA-A, -B, and -C haplotypes were estimated using a computer program developed at the 11th International Histocompatibility Workshop (12). The frequencies of HLA-A, -B, -C, and -DR antigens of the patients were compared with those of 200 healthy control subjects and the frequencies of HLA haplotypes of the patients were compared with those obtained from a study of 107 Korean families (13).

Statistical Analysis

Statistical analysis of HLA antigen and haplotype frequencies was performed using the chi-square test (or Fisher exact test when indicated) with the SAS program.

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

HLA-A, -B, -C, and -DR Antigens

The phenotype frequencies of HLA-A, -B, and -C locus antigens are shown in Table 1. The most remarkable finding was increased frequency of HLA-A11 in patients with DPB compared to the control subjects (53.3% versus 17.5%, corrected p [pc] = 0.00012, odds ratio [OR] = 5.4). Increased frequencies of HLA-B55 (16.7% versus 3.5%, pc = 0.05, OR = 5.5), B62 (36.7% versus 16%, p < 0.01, OR = 3.0), and Cw4 (23.3% versus 8.5%, p < 0.05, OR = 3.3) were also observed in the patients compared with the control subjects. Increases of A11 and B55 were statistically significant after correction for the number of antigens compared. For HLA-DR (data not shown), the frequency of HLA-DR13 was decreased in the patients compared with the control subjects (5.0% versus 25.5%, p = 0.05, OR = 0.15).

HLA-A, -B, and -C Haplotypes

HLA haplotype frequencies of A-C, C-B, A-B, and A-C-B loci are shown in Table 2. Significant increases in the frequencies of A11-associated haplotypes were observed in the patients compared with the control subjects. Most remarkable changes in the patients were increases of A11-Cw1 (16.0% versus 1.9%, pc = 6.3 × 10⁷, OR = 12.1), Cw1-B55 (7.9% versus 1.0%, pc = 0.004, OR = 9.8), and A11-B62 (12.9% versus 2.2%, pc = 5.5 × 10⁴, OR = 7.9) haplotypes. These haplotypes (A11-Cw1, Cw1-B55, A11-B62) showed much stronger positive association with DPB compared with HLA-A11, B55, or B62 antigens themselves, and the association was highly significant after correction for the number of haplotypes compared. Although the frequency of HLA-B54 antigen itself was not increased, the frequencies of A11-B54 (p = 0.006) and A11-Cw1-B54 (p = 0.013) haplotypes were significantly increased in the patients compared with the control subjects.

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Although DPB has been well established as a distinct clinical entity, the pathogenesis of the disease is not well known. Observations of increase in the frequency of HLA-B54 in Japanese patients (9, 10) and occurrence of familial cases (14, 15) suggest possible genetic predisposition to the disease susceptibility. HLA-B54 antigen is unique to East Asian ethnic groups and is absent or very rare in other ethnic groups. The allele frequency of B54 is highest among Japanese and Koreans (6.3% and 6.5%, corresponding to antigen frequency of 12.2% and 12.6%, respectively) and lower in Chinese (2.4-4.9%) and other mongoloids (< 3%) (11). Besides, Japanese and Koreans are closest among all ethnic groups based on the distribution of HLA antigens and haplotypes (13, 16). If HLA-B54 antigen itself is involved in the pathogenesis or disease susceptibility to DPB, we could expect a similar HLA association in Korean patients with DPB.

In the present study of Koreans, we have found striking ethnic differences in the association of HLA class I antigens with DPB between Koreans and Japanese. The frequency of HLA-B54, which had been reported to have a strong positive association with DPB in Japanese (9, 10), was not different between the patient and control groups in Koreans (13.3% versus 12.5%). To understand better the nature of HLA association with the disease susceptibility to DPB, HLA changes were compared between Korean and Japanese patients in Table 3. In two different studies of Japanese patients (9, 10), the most significant and consistent change was increased frequency of B54. In Korean patients, the most striking change was increased frequency of A11. HLA-B55 appeared to have an equivalent degree of positive association with the disease in Japanese (9) and Korean patients. Other HLA antigens showing weak or variable associations were A11 and Cw1 in Japanese and B62 and Cw4 in Korean patients. To evaluate the possible association of certain HLA haplotypes with the disease, HLA class I haplotypes in Korean DPB patients were analyzed (Tables 2 and 3). Various A11-associated haplotypes were significantly increased in the patients compared with the control subjects. Some of the haplotypes (A11-Cw1, Cw1-B55, A11-B62) showed much stronger positive association with DPB compared with HLA-A11, B55, or B62 antigens themselves.

Two different hypotheses can be postulated for the mechanisms involved in the association of HLA class I antigens with the susceptibility to DPB. The first one is that an etiologic agent utilizes HLA antigen itself as an immunologic target for the pathogenesis of the disease. And ethnic differences in HLA association arise from a different distribution of HLA antigens involved in the pathogenesis of the disease. This is unlikely due to the fact that HLA-A11 and B54 antigens, showing the strongest positive association with the disease in Korean and Japanese patients, respectively, have quite similar distribution in both ethnic groups (11). The second hypothesis is that the association of HLA antigens with the disease represent linkage disequilibria to the same disease gene(s) even if different HLA antigens are identified as risk factors in different ethnic groups. The latter view is supported by the differences and similarities in the HLA association with DPB between Korean and Japanese patients. Other strong evidence supporting this view could be obtained from the results of haplotype analysis in the present study (Tables 2 and 3). Some of the A11-associated haplotypes showed much stronger positive association with the disease compared with A11 antigen itself. HLA-B54 and Cw1 antigens showed positive association with the disease only in Japanese patients, and not in Koreans. However, it is of interest that A11-associated haplotypes involving these antigens (A11-B54, A11-Cw1) showed significant positive association in Korean patients. From these findngs, the disease gene(s) involved in DPB is supposed to be located between HLA-A and HLA-B loci, showing different linkage disequilibria with HLA-A and HLA-B genes among Koreans and Japanese.

From the results of HLA studies in DPB patients, it is suggested that HLA class I genes are associated with the disease in both Koreans and Japanese. The HLA antigens showing the strongest positive association with DPB are A11 in Koreans and B54 in Japanese. In addition, HLA haplotype analysis in Koreans show that certain HLA-A11-associated haplotypes have even stronger positive association with the disease, compared with A11 antigen itself. It can be concluded that the disease gene(s) involved in DPB is located within the HLA class I region, most probably between HLA-A and HLA-B loci. Further collaborative studies between Koreans and Japanese are needed to search for the candidate genes associated with the disease.