湖南地区HBV患者MHC-Ⅰ类链相关基因A第五外显子微卫星多态性研究

分别提取湖南汉族人群108例慢性乙肝病毒(hepatitis B virus,HBV)患者、96例HBV携带者和142例健康对照者外周血基因组DNA,利用聚合酶链反应和基因扫描技术分别对它们的主要组织相容性复合体Ⅰ类链相关A(major histocompatibility complex class Ⅰ chain related A,MICA)基因第5外显子进行微卫星多态性分析;应用DNA测序分析对不同的基因型进行验证,同时应用PCR/SSP技术进行MICA*Del检测,确定MICA基因第5外显子基因型.发现本研究的三组中分别检出A4、A5、A5.1、A6、A9五种等位基因,且以A5和A5.1为主;在HBV携带组和健康对照组中分别检出MICA* Del基因.结果同时显示慢性HBV患者组MICA*A5.1/A9基因型频率、慢性HBV患者组的MICA*A9等位基因频率、慢性HBV患者组MICA* A9表型频率和HBV携带组MICA*A5.1/A9基因型频率均低于相应的健康对照组;而湖南地区汉族人群HBV携带者和慢性HBV患者间的基因型频率、等位基因频率和表型频率无显著性差异;因而推测MICA*A5.1/A9基因型和MICA*A9等位基因可能是抗HBV感染的一种保护性等位基因.为今后进一步研究HBV的感染、预防和治疗提供参考依据.     

乙醇代谢酶基因多态性与酒精性肝病关系

目的:研究乙醇代谢酶基因ADH3、ALDH2及CYP2E1 RsaⅠ/PstⅠ位点多态性与酒精性肝病(ALD)的关系。探讨遗传因素对其易感性的影响。方法:PCR-RFLP法检测东北地区汉族男性健康者、嗜酒者及ALD患者中ADH3、ALDH2和CYP2E1的基因型并比较其基因频率。结果:ADH3和CYP2E1的两种等位基因在各组中的分布差异无统计学意义;ALDH2的两种等位基因在三组间的频率存在统计学差异,ALDH2~*1在嗜酒及ALD组中的频率显著高于其在对照组中的频率,二组均以ALDH2~*1/2~*1基因型为主,未见ALDH2~*2/2~*2;与嗜酒者相比,ALDH2~*2在ALD组中的频率显著提高,且均为ALDH2~*1/2~*2。结论:ADH3及CYP2E1 RsaⅠ/PstⅠ位点多态性与东北地区汉族男性ALD的发生无关;ALDH2~*1是嗜酒的诱因,ALDH2~*2/2~*2可防止嗜酒及ALD的发生,ALDH2~*1/2~*2具有最高的患病风险。     

中国汉族人群肿瘤坏死因子-α（TNF-α）基因-238G/A和-308G/A多态性与强迫症的关联分析（英文）

目的：探讨在中国汉族人群中强迫症与TNF-a基因-238G/A和-308G/A多态性之间的关联。方法：我们的研究所招募的161例强迫症患者和325名健康对照中,应用PCR-RFLP比较了OCD组和对照组之间的TNF-α基因在-238G/A（rs361525）和-308G/A（rs1800629）位点的基因型和等位基因频率多态性。结果：在中国大陆汉族人群TNF-α基因的OCD组与对照组之间-308 G/A等位基因频率及-238G/A的基因型频率和等位基因频率无显着差异,而-308G/A基因型频率有显著不同。在-308G/A位点,女性强迫症患者和对照组之间的基因型频率关联分析有增高的趋势。结论：我们的研究结果表明,肿瘤坏死因子-α在-308G/A点位多态性可能会影响在中国大陆汉族人群强迫症的发展。     

中国汉族人群肿瘤坏死因子-α(TNF-α)基因-238G/A和-308G/A多态性与强迫症的关联分析

目的:探讨在中国汉族人群中强迫症与TNF-a基因-238G/A和-308G/A多态性之间的关联.方法:我们的研究所招募的161例强迫症患者和325.名健康对照中,应用PCR-RFLP比较了OCD组和对照组之间的TNF-α基因在-238G/A(rs361525)和-308G/A(rs1800629)位点的基因型和等位基因频率多态性.结果:在中国大陆汉族人群TNF-α基因的OCD组与对照组之间-308 G/A等位基因频率及-238G/A的基因型频率和等位基因频率无显着差异,而-308G/A基因型频率有显著不同.在-308G/A位点,女性强迫症患者和对照组之间的基因型频率关联分析有增高的趋势.结论:我们的研究结果表明,肿瘤坏死因子-α在-308G/A点位多态性可能会影响在中国大陆汉族人群强迫症的发展.     

乳腺癌与MICA基因多态性的相关性研究

目的:研究海南汉族人群MICA等位基因的多态性与乳腺癌的相关性。方法:采用PCR-SSP和PCRSBT方法对样本MICA等位基因的多态性进行检测分析。结果:乳腺癌患者中有检测出10种MICA等位基因,其中MICA*002/019基因型频率较对照组显著偏低(OR=0.32,Pc0.05)。结论:MICA*002/019基因型可能与乳腺癌的保护相关。     

健康人群肿瘤坏死因子-α基因多态性的分析

了解汉族健康人群中TNF-A基因多态性的分布,研究TNF-α表达与相关疾病之间的联系。采用PCR-限制性长度片段多态分析法检测140名重庆地区汉族健康人群的TNF—A-308,TNF—A-857位点基因多态性,计算其基因型和等位基因频率,结果显示TNF-A-308G／G、G／A、4朋基因型的频率分别为89％、11％、1％,其等位基因的发生频率以G等位基因最常见（93％）,其次为A等位基因（7％）。TNF—A-857C／C、C／T、形,基因型的频率分别为68％、36％、8％,其等位基因发生频率以C等位基因最常见（81％）,其次为T等位基因（19％）。由结果可以得出重庆地区汉族健康人群TNF—A-308位点存在G／A多态性,TNF-A-857位点存在C／T多态性。     

贵州荔波封闭人群MTHFR基因多态性研究分析

目的 对贵州汉族、布依族亚甲基四氢叶酸还原酶（Methylenetetrahydrofolate Reductase,MTHFR）基因多态性进行研究,为贵州少数民族基因多态性数据库的建立提供相关数据。方法 应用聚合酶链式反应及限制性片段长度多态性检测贵州荔波汉族90例、布依族119例MTHFR基因两个单核苷酸（677及1298位）多态位点的基因频率及基因型频率。结果 汉族、布依族MTHFR 677位T等位基因的分布频率分别是22、8％,16．1％,x^2=1．561,P〉0．1;MTHFR 1298位C等位基因的分布频率分别是28．9％,39、1％,x^2=2．075,P〉0．1;677CT／1298AC双杂合子的分布频率分别是16．66％,22．7％。结论 MTHFRC 677T和A1298C多态性在中国南方和北方人群存在群体差异;贵州汉族与布依族此两位点无显著性差异。贵州荔波布依族MTHFR 1298位有较高的C等位基因频率。     

湖南汉族人群MICB等位基因多态性与白血病的相关性研究

目的:研究湖南汉族人群MICB等位基因的多态性与白血病的相关性。方法:MICB等位基因分型用PCR-SSP和PCR-SBT的方法。结果:白血病患者中有13种MICB等位基因被检测出,其中MICB*005:02/010基因频率较对照组显著偏低(OR=0.416,P<0.05)。不同类别白血病之间MICB等位基因多态性没有显著差异。结论:MICB*005:02/010等位基因可能与白血病的保护相关。     

多巴胺转运体基因多态性与癌症患者性别关系研究

目的:探讨多巴胺转运体基因(DAT1)多态性与新疆汉族癌症患者性别的关系.方法:采用聚合酶链式反应和VNTR多态性分析技术对新疆汉族癌症患者DAT1多态性进行检测,比较各组间等位基因和基因型频率分布的差异及组内男女性别等位基因和基因型频率分布的差异.结果:在213例无关癌症人群个体中,DATI VNTR多态性表现出6～12倍重复的7种等位基因,共检出7种基因型.男女癌症患者的DATl VNTR等位基因及基因型频率的差异均无显著性(P>0.05).结论:多巴胺转运体基因(DAT1)3'端40bp可变串联重复多态性可能与汉族癌症患者的性别无关.     

多巴胺D2受体基因TaqIB多态性与酒依赖的关系

目的:探讨汉族人群中多巴胺D2受体(DRD2)基因TaqIB多态性与酒依赖的相关性.方法:采用聚合酶链式反应-限制性片断长度多态性(PCR-RFLP)技术,检测酒依赖组(80例)和对照组(95例)的DRD2基因TaqIB多态性的基因型和等位基因频率.结果:酒依赖组和时照组的DRD2基因TaqIB多态性的基因型和等位基因频率有显著性差异,等位基因B2的携带者显著降低其嗜酒的发生率(OR:1.636,P＜0.05).结论:本研究提示,在汉族人群中DRD2基因TaqIB多态性与酒依赖存在相关性,TaqIB2等位基因可能是降低酒依赖发病的影响因子.     

应用PCR-SSOP技术研究中国汉族、维吾尔族HLA-A基因座基因多态性

人类白细胞抗原（Human Leukocyte Antigen,HLA)基因复合物位于6p21.3,有220多个不同的功能基因,是人类基因组最复杂的遗传多态系统。HLA等位基因的变异在医学、法医学、人类学等领域具有重要的意义。自从1964年以来,HLA分型一直采用经典的微量淋巴细胞毒实验,但该方法是血清学水平的分,不能识别很多特异性的等位基因,而且高质量的抗体也不易获得。从20世纪90年代起,在国家自然科学基金的资助下,首先开展HLAⅡ类位点基因分研究及大规模群体多态性调查,所获得的中国主要民族基因数据已应用于多个领域。相比之下,HLAⅠ类基因数量更丰富,包含了A、B、C、E、F、G和假基因H、J、K、L等10个位点;基因分子结构更复杂,更具多态性。因此,HLAⅠ类DNA分型比HLAⅡ类分型及行多困难。直至目前中国人群HLA－A基因座基因多态性和分布频率的研究尚未充分进行。而任何DNA标记用于遗传分析、法医鉴定等领域之前,必须先进行群体调查,建立不同民族基因数据库,这是不可逾越的基础工作。鉴于此,采用灵敏而非同位素污染的PCR－SSOP基因分型技术,对165个汉族和162个维吾尔族个体的HLA－A基因座多态性进行调查。结果在汉族群体中发现22种等位基因,频率最高的是HLA－A＊1101（19．7％）,其次是＊201（12．72％）;在维族群体中发现22种等位基因,频率最高的是＊2407（17．90％）,等位基因＊0101、＊0201和＊3301的频率均大于10％;HLA－A＊0203、＊0205、＊0302、＊2403和＊3302仅在汉族群体中检出;HLA－A＊0205、＊0211、＊2301、＊2502、＊68012和＊6802仅在维族群体中检出。按照Hardy-Weinberg平衡定律检验,两个民族各等位基因型频率的预期值与实际观察值相吻合（P＞0．05）,证明了所获得汉族、维吾尔族HLA－A位点基因频率具有可靠性;同时也表明各等位基因的遗传特征符合符合孟德尔规律。经计算机统计分析,汉族群体HLA－A基因座杂合度（Heterozygosity,H)、个体识别率（Discrimination Power,DP)和非父排排率（Proba-bility of Paternity Exclusion,EPP)分别为0．9029、0．9776和0．8592;维族群体H、DP和EEP分别为0．9063、0．9379和0．7885。和其他遗传标记（如VNTR、STR、SNP）的单一位点相比,HLA－A具有高度的杂合率、个体识别率和非父排除率。因此,HLA－A等位基因在法医个体识别、亲权鉴定、基因诊断、人类学等领域具有重要的应用价值。     

Polymorphisms in MHC class Ia genes and resistance to IHNV in rainbow trout (Oncorhynchus mykiss)

Infectious haematopoietic necrosis virus (IHNV) is detrimental to the farming of rainbow trout (Oncorhynchus mykiss) and other salmonids in the Northern hemisphere. The major histocompatibility complex (MHC) plays a key role in immune response in invertebrates, as evidenced by the close correlation of MHC polymorphisms with disease resistance/susceptibility. To analyse the correlation between rainbow trout resistance and susceptibility to IHNV and genetic variation in exon 2 of MHC class Ia gene, UBA, we employed two approaches, namely, polymerase chain reaction-single strand conformation polymorphism analysis and cloning/sequencing. From 102 resistant and 82 susceptible individuals, a total of 12 alleles in UBA exon 2 (GenBank: JX136662–JX136673) were identified, including 11 novel alleles. The maximum number of these alleles in a single individual was four, suggesting that UBA exon 2 most likely resides on at least two loci in the genome. Most of the variations in UBA exon 2 were located in the peptide-binding region and were determined to have been subject to positive selection during evolution. Correlation analysis revealed that Onmy-UBA*0111 and Onmy-UBA*0107 are highly associated with IHNV susceptibility (P = 0.001), whereas Onmy-UBA*0101, Onmy-UBA*0102, and Onmy-UBA*0103 are highly related to IHNV resistance (P = 0.000). In addition, the three resistant alleles were predominant in the IHNV disease-resistant population; thus, these molecular markers can be used for anti-IHNV breeding of rainbow trout.     

HLA class II gene and haplotype diversity in the population of the island of Hvar, Croatia

The DRB1, DRB3, DRB5, DQA1 and DQB1 allele polymorphisms were analysed in 3 western and 3 eastern villages of the island of Hvar using PCR-SSOP method and 12th International Workshop primers and probes. Three DQB1 alleles (*0304, *0305, *0607) detected in the population of the island of Hvar (HP) have not yet been observed in general Croatian population (GCP). Significant differences were observed between two regions of Hvar for: a) DRB1*0701 allele (p < 0.001), b) DQA1*0201 allele (p < 0.01), and c) DRB1*0101-DQA1*0101-DQB1*0501 haplotypic association (p < 0.05). Two unusual haplotypic associations, which have not yet been described in general Croatian population (GCP), DRB1*0101-DQA1*0102-DQB1*0501 and DRB1*1501-DQA1 *0102-DQB1*0604 were observed in the population from the island of Hvar (HP). Measures of genetic kinship and genetic distances revealed isolation and clusterization which coincides with the known ethnohistorical, as well as biological and biocultural data obtained from a series of previous investigations. The five studied village subpopulations formed two clusters (East-West) to which the far eastern village (with the highest rii of 0.0407) joined later, thus indicating possible impact of historical immigrations from the mainland.     

用PCR-RFLP方法研究藏族HLA-DQA1和-DQB1基因多态性

应用目前HLA研究领域中成熟的,有效的PCR-RFLP基因分型技术,从DNA水平对藏族健康群体进行了HLA-DQA1(49人)和-DQB1(49人)基因分型,这在国内外属首次。所采用的PCR-RFLP基因分型技术是在HLA-DQA1和-DQB1各等位基因全部序列已知的情况下,对其第2个外显子碱基序列扩增进而进行RFLP分析的方法。这种方法得到的RFLP的所有片段都是已知序列,因而精确度很高,同时为发现新的等位基因提供了成熟而有效的分析方法。研究结果表明,在藏族DQA1的8个等位基因,DQA1*0301的基因频率最高(36.74％)。DQA1*0601(4.08％)、*0103(4.08％)和*0401(5.10％)最低。在DQB1的16个等位基因中,OQB1*0302(16.33％)、*0303(15.31％)和*0602(15.31％)为最常见,没有观察到*0504。统计分析表明,在DQA1各等位基因分布上,藏族与新疆汉族、北方汉族、上海汉族十分相近;与维吾尔族和哈萨克族也没有明显差异。在OQB1各等位基因的分布上,藏族与汉族、维族、哈族之间略有差异,而汉族、维族、哈族之间也存在一些差异。     

An extended HLA-DQ-DR haplotype rather than DRB1 alone contributes to RA predisposition

 In the present study, we tested our hypothesis on the role of a DQ-DR haplotype in rheumatoid arthritis (RA) predisposition. Using two groups of patients and controls, one from The Netherlands and one from Switzerland, we found that DQA1*0301-homozygous and DQA1*0301//DQA1*0101/04-heterozygous individuals are highly predisposed to RA in both populations, while DQA1*0101/04-homozygous are not. The DQA1*0301-DRB1*0403/06/07 and DQA1*0301-DRB1*0901 haplotypes are not associated with RA by themselves but strongly increase the risk of developing disease in DQA1*0301- and DQA1*0101/04-heterozygous. DRB1 alleles carrying the motif DERAA in their third hypervariable region, i.e., *0103, *0402, *1102, *1103, *1301, and *1302, provide a long-lasting protection against RA in DQA1*0101/04- but not in DQA1*0301-positive individuals. These data show that considering both DQ and DR gives a better distinction between patients and controls than the shared epitope hypothesis. Received: 5 March 1998 / Revised: 21 April     

HLA class II DR and DQ genotypes and haplotypes associated with rheumatic fever among a clinically homogeneous patient population of Latvian children

The HLA system is being paid more and more attention because it is very significant in polymorphous immunological reactions. Several studies have suggested that genetic susceptibility to rheumatic fever (RF) and rheumatic heart disease (RHD) is linked to HLA class II alleles. We hypothesized that HLA class II associations within RHD may be more consistent if analysed amongst patients with a relatively homogeneous clinical outcome. A total of 70 RF patients under the age of 18 years were surveyed and analysed in Latvia. HLA genotyping of DQA1, DQB1 and DRB1 was performed using PCR with amplification with sequence-specific primers. We also used results from a previous study of DQB1 and DRB1 genotyping. In the RF patients, HLA class II DQA1*0401 was found more frequently compared to DQA1*0102. In the RF homogeneous patient groups, DQA1*0402 has the highest odds ratio. This is also the case in the multivalvular lesion (MVL) group, together with DQA1*0501 and DQA1*0301. In the chorea minor patients, DQA1*0201 was often found. Significant HLA DQA1 protective genotypes were not detected, although DQA1 genotypes *0103/*0201 and *0301/*0501 were found significantly and frequently. In the distribution of HLA DRB1/DQA1 genotypes, *07/*0201 and *01/*0501 were frequently detected; these also occurred significantly often in the MVL group. The genotype *07/*0201 was frequently found in Sydenhamn's chorea patients that had also acquired RHD, but DRB1*04/DQA1*0401 was often apparent in RF patients without RHD. In the distribution of HLA DQA1/DQB1 genotypes, both in RF patients and in the homogeneous patient groups, the least frequent were *0102/*0602-8. The genotype DQA1*0501 with the DQB1 risk allele *0301 was often found in the MVL group. The genotype *0301/*0401-2 was frequently found in the RF and Sydenhamn's chorea patient groups. The haplotype *07-*0201-*0302 was frequently found in RF and homogeneous patient groups, including the MVL group. In addition, haplotypes *04-*0401-*0301 and *04-*0301-*0401-2 were frequent amongst patients with Sydenhamn's chorea. The protective alleles DQA1*0102 and DQB1*0602-8 in the haplotype DRB1*15 were less frequently found in RF patients. The results of the present study support our hypothesis and indicate that certain HLA class II haplotypes are associated with risk for or protection against RHD and that these associations are more evident in patients in clinically homogeneous groups.     

HLA-DQ-restricted CD4+ T cells specific to streptococcal antigen present in low but not in high responders.

We have found that the low immune response to streptococcal cell wall Ag (SCW) was inherited as a dominant trait and was linked to HLA, as deduced from family analysis. In the present report, HLA class II alleles of healthy donors were determined by serology and DNA typing to identify the HLA alleles controlling low or high immune responses to SCW. HLA-DR2-DQA1*0102-DQB1*0602(DQw6)-Dw2 haplotype or HLA-DR2-DQA1*0103-DQB1*0601(DQw6)-DW12 haplotype was increased in frequency in the low responders and the frequency of HLA-DR4-DRw53-DQA1*0301-DQB1*0401(DQw4)-Dw15 haplotype or HLA-DR9-DRw53-DQA1*0301-DQB1*0303(DQw3)-Dw23 haplotype was increased in the high responders to SCW. Homozygotes of either DQA1*0102 or DQA1*0103 exhibited a low responsiveness to SCW and those of DQA1*0301 were high responders. The heterozygotes of DQA1*0102 or 0103 and DQA1*0301 showed a low response to SCW, thereby confirming that the HLA-linked gene controls the low response to SCW, as a dominant trait. Using mouse L cell transfectants expressing a single class II molecule as the APC, we found that DQw6(DQA1*0103 DQB1*0601) from the low responder haplotype (DR2-DQA1*0103-DQB1*0601(DQw6)-Dw12) activated SCW-specific T cell lines whereas DQw4(DQA1*0301 DQB1*0401) from the high responder haplotype (DR4-DRw53-DQA1*0301-DQB1*0401(DQw4)-Dw15) did not activate T cell lines specific to SCW. However, DR4 and DR2 presented SCW to CD4+ T cells in both the high and low responders to SCW, hence the DR molecule even from the low responder haplotype functions as an restriction molecule in the low responders. Putative mechanisms linked to the association between the existence of DQ-restricted CD4+ T cells specific to SCW, and low responsiveness to SCW are discussed.     

Sex influences on the penetrance of HLA shared-epitope genotypes for rheumatoid arthritis.

The association between rheumatoid arthritis (RA) and HLA DRB1 alleles may arise through linkage disequilibrium with a disease locus or the direct involvement of HLA alleles in RA. In support of the latter possibility, the shared-epitope hypothesis has been postulated, stating that conformationally similar DR beta chains encoded by several DRB1 alleles confer disease susceptibility. To examine these alternative hypotheses of marker-disease association and to investigate gender differences in RA susceptibility, we analyzed the distributions of PCR-based DRB1 genotypes of 309 Caucasian RA patients and 283 Caucasian controls. Initially, the marker-association-segregation chi 2 method was used to evaluate evidence for linkage disequilibrium and the direct involvement of markers DR4 Dw4, DR4 Dw14, and DR1 in RA susceptibility. Additional shared-epitope models that grouped DRB1 alleles into five classes (*0401, *0404/*0102, *0405/*0408/*0101, *1001, and all others) and postulated relationships between genotypes and RA susceptibility were also fitted to observed genotypic distributions by the method of minimal chi 2. For females, a linkage-disequilibrium model provided a good fit to the data, as did a shared-epitope model with RA most penetrant among individuals with the *0401,*0401 genotype. For males, the best model indicated highest RA penetrance among shared-epitope compound heterozygotes. Clinically, male RA patients had more subcutaneous nodules and greater use of slowly acting antirheumatic drugs, while female RA patients had earlier disease onset. This study therefore suggests that sex-related factors influence the RA penetrance associated with DRB1 shared-epitope genotypes and that DRB1 effects on RA prognosis and pathogenesis should be considered separately for men and women.     

MHC allele-specific binding of a malaria peptide makes it become promiscuous on fitting a glycine residue into pocket 6

Peptide 1585 (EVLYLKPLAGVYRSLKKQLE) has a highly conserved amino-acid sequence located in the Plasmodium falciparum main merozoite surface protein (MSP-1) C-terminal region, required for merozoite entry into human erythrocytes and therefore represents a vaccine candidate for P. falciparum malaria. Original sequence-specific binding to five HLA DRB1* alleles (0101, 0102, 0401, 0701, and 1101) revealed this peptide's specific HLA DRB1*0102 allele binding. This peptide's allele-specific binding to HLA DRB1*0102 took on broader specificity for the DRB1*0101, -0401, and -1101 alleles when lysine was replaced by glycine in position 17 (peptide 5198: EVLYLKPLAGVYRSLKG(17)QLE). Binding of the identified G(10)VYRSLKGQLE(20) C-terminal register to these alleles suggests that peptide promiscuous binding relied on fitting Y(12), L(15), and G(17) into P-1, P-4, and P-6, respectively. The implications of the findings and the future of this synthetic vaccine candidate are discussed.     

Biased T-cell receptor usage is associated with allelic variation in the MHC class II peptide binding groove

 A comprehensive analysis was carried out of the tri-molecular complex of peptide, major histocompatibility class II molecule, and T-cell receptor (TcR) involved in the recognition of the promiscuous HA (306–318) peptide, restricted by one of two closely related HLA-DR alleles, HLA-DRB1*0101 and HLA-DRB1*0103. These two DR molecules differ by only three amino acids at positions 67, 70, and 71, in the third variable region of the DRB1 chain. None of the HA (306–318)-specific T-cell clones restricted by these two DR molecules tolerated amino acid substitution at the peptide-binding position 71, despite the fact that the substitution did not interfere with peptide binding. The majority of the DRB1*0103-restricted clones tolerated substitution of the amino acid at the TcR-contacting position 70, while the DRB1*0101-restricted T cells did not. Biased usage of TRVA and TRVB segments was observed for the DRB1*0103-restricted clones; in contrast, apparently random usage was seen in the DRB1*0101-restricted T cells. Finally, limiting dilution analysis revealed a lower frequency of T cells reactive with the HA peptide in a DRB1*0103 compared with a DRB1*0101 individual. Taken together these data suggest that biased TcR gene usage may reflect a relatively low precursor frequency of T cells, and the need for clonal expansion of a limited set of high avidity T cells. Received: 7 August 1998 / Revised: 19 November 1998