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for iTTP.60 Recently, additional evidence for the involve- ment of the HLA-DRB1*11 and HLA-DQB1*03 haplotype in the development of iTTP was provided from a case of familial iTTP on 2 first-degree relatives (mother and daughter) who both carried HLA-DRB1*1101/DRB1*1104 and the linked HLA-DQB1*03 allele.
Interestingly, previous reports had shown associations between DRB1*11 and certain clinical conditions such as systemic sclerosis, early-onset juvenile chronic arthritis and sarcoidosis. In accordance with these findings, such associated conditions could be observed in our iTTP reg- istry.61,62 The wide spectrum of autoimmune diseases shar- ing DRB1*11 clearly argues for the existence of additional risk factors, which may determine the specific clinical fea- tures of those diseases.
Mancini et al. employed immunochip analysis to identi- fy susceptibility loci in the HLA region for iTTP.63 A com- mon variant rs6903608 was shown to confer 2.6-fold increased risk of development of iTTP.63 The rs6903608 is an intron of pseudogene HLA-DRB9 that maps to an inter- genic region between HLA-DRA and HLA-DRB5 (Figure 1). Unexpectedly, HLA-DRB1*11 and HLA-DQB1*03 were not identified as risk factors in this study. It was pro- posed that rs6903608 is in linkage disequilibrium with HLA-DRB1*11, thereby masking a potential contribution of this allele.63 Interestingly, imputation analysis suggested that HLA-DQB1*0503 was also associated with iTTP.63 This particular allele had not been linked to iTTP in previ- ous studies.
Potential mechanisms of HLA association with onset of iTTP
The observed association between HLA alleles and the development of autoimmunity has been considered to arise from recognition of self-peptides by low affinity CD4+ T cells that have escaped negative selection in the thymus.64 It has recently been shown that differences in the intrinsic stability of HLA-DQ proteins may be linked to the onset of autoimmunity.65 Several HLA-DQ proteins have been shown to be poorly expressed on the cell sur- face; immuno-dominant epitopes that may potentially bind to these HLA-DQ proteins may, therefore, not be sufficiently presented, allowing CD4+ T cells to escape the negative selection in the thymus. This would result in the appearance of potentially auto-reactive CD4+ T cells in the periphery that could contribute to the onset of autoimmu- nity.65 It is possible that the increased frequency of HLA- DQB1*03 in patients with iTTP is caused by the inability of HLA-DQB1*03-containing DQ proteins to efficiently present immune-dominant self-peptides. This would result in a defective elimination of self-reactive CD4+ T cells during negative selection in the thymus. In this respect, the single nucleotide polymorphism (rs6903608) that has been linked to the onset of iTTP (see above), is located in the HLA locus that affects expression of a num- ber of MHC class II subunits.63 More precisely, this single nucleotide polymorphism is located in close proximity to the DRB9 pseudogene, upstream of DRA gene. Although this has not been established, the rs6903608 single
Figure 1. The genes of the human MHC locus involved in immune-mediated thrombotic thrombocytopenic purpura (iTTP) susceptibility. Localization of genes of the two HLA alleles, HLA- DQB1*03 and HLA-DRB1*11, previously described as risk factors for iTTP, as well as the approximate localization of single polymorphism rs6903608, which appears to be in linkage dise- quilibrium with HLA-DRB1*11 and which has also been linked to the onset of iTTP.97
haematologica | 2018; 103(7)