Role of a Novel Human Leukocyte Antigen-DQA1*01:02;DRB1*15:01 Mixed Isotype Heterodimer in the Pathogenesis of "Humanized" Multiple Sclerosis-like Disease.

Gene-wide association and candidate gene studies indicate that the greatest effect on multiple sclerosis (MS) risk is driven by the HLA-DRB1*15:01 allele within the HLA-DR15 haplotype (HLA-DRB1*15:01-DQA1*01:02-DQB1*0602-DRB5*01:01). Nevertheless, linkage disequilibrium makes it difficult to define, without functional studies, whether the functionally relevant effect derives from DRB1*15:01 only, from its neighboring DQA1*01:02-DQB1*06:02 or DRB5*01:01 genes of HLA-DR15 haplotype, or from their combinations or epistatic interactions. Here, we analyzed the impact of the different HLA-DR15 haplotype alleles on disease susceptibility in a new "humanized" model of MS induced in HLA-transgenic (Tg) mice by human oligodendrocyte-specific protein (OSP)/claudin-11 (hOSP), one of the bona fide potential primary target antigens in MS. We show that the hOSP-associated MS-like disease is dominated by the DRB1*15:01 allele not only as the DRA1*01:01;DRB1*15:01 isotypic heterodimer but also, unexpectedly, as a functional DQA1*01:02;DRB1*15:01 mixed isotype heterodimer. The contribution of HLA-DQA1/DRB1 mixed isotype heterodimer to OSP pathogenesis was revealed in (DRB1*1501xDQB1*0602)F1 double-Tg mice immunized with hOSP(142-161) peptide, where the encephalitogenic potential of prevalent DRB1*1501/hOSP(142-161)-reactive Th1/Th17 cells is hindered due to a single amino acid difference in the OSP(142-161) region between humans and mice; this impedes binding of DRB1*1501 to the mouse OSP(142-161) epitope in the mouse CNS while exposing functional binding of mouse OSP(142-161) to DQA1*01:02;DRB1*15:01 mixed isotype heterodimer. This study, which shows for the first time a functional HLA-DQA1/DRB1 mixed isotype heterodimer and its potential association with disease susceptibility, provides a rationale for a potential effect on MS risk from DQA1*01:02 through functional DQA1*01:02;DRB1*15:01 antigen presentation. Furthermore, it highlights a potential contribution to MS risk also from interisotypic combination between products of neighboring HLA-DR15 haplotype alleles, in this case the DQA1/DRB1 combination.

Uptake and presentation of myelin basic protein by normal human B cells.

B cells may play both pathogenic and protective roles in T-cell mediated autoimmune diseases such as multiple sclerosis (MS). These functions relate to the ability of B cells to bind and present antigens. Under serum-free conditions we observed that 3-4% of circulating B cells from healthy donors were capable of binding the MS-associated self-antigen myelin basic protein (MBP) and of presenting the immunodominant peptide MBP85-99, as determined by staining with the mAb MK16 recognising the peptide presented by HLA-DR15-positive cells. In the presence of serum, however, the majority of B cells bound MBP in a complement-dependent manner, and almost half of the B cells became engaged in presentation of MBP85-99. Even though complement receptor 1 (CR1, CD35) and CR2 (CD21) both contributed to binding of MBP to B cells, only CR2 was important for the subsequent presentation of MBP85-99. A high proportion of MBP85-99 presenting B cells expressed CD27, and showed increased expression of CD86 compared to non-presenting B cells. MBP-pulsed B cells induced a low frequency of IL-10-producing CD4+ T cells in 3 out of 6 donors, indicating an immunoregulatory role of B cells presenting MBP-derived peptides. The mechanisms described here refute the general assumption that B-cell presentation of self-antigens requires uptake via specific B-cell receptors, and may be important for maintenance of tolerance as well as for driving T-cell responses in autoimmune diseases.

Ficolin-2 recognizes DNA and participates in the clearance of dying host cells.

Ficolin-2 is a serum opsonin, which has been shown to be a pattern recognition molecule in the lectin complement activation pathway. Because innate immune mechanisms are involved in maintaining tissue homeostasis we hypothesized that Ficolin-2 also participate in the clearance of dying host cells. We found that Ficolin-2 binds to late apoptotic cells, as well as to apoptotic bodies and necrotic cells, but not to early apoptotic cells. We demonstrated that Ficolin-2 binds DNA in a calcium dependent manner and that DNA inhibits the binding to late apoptotic and necrotic cells, suggesting that DNA on permeable dying cells is a plausible ligand. Reconstituting serum deficient of Ficolin-2, C1q and mannose-binding lectin with Ficolin-2 augmented deposition of complement C4 on necrotic cells. Opsonization leads to an enhanced attachment/uptake of necrotic cells by macrophages. In conclusion dying host cells expose ligands with the capacity of binding Ficolin-2, which in turn leads to increased attachment and engulfment. Binding of Ficolin-2 to DNA points at nucleic acid exposed by permeable late apoptotic and necrotic cells as one of the ligands for Ficolin-2. Ficolin-2 may therefore be a scavenger molecule participating in the removal of host cells and maintenance of tissue homeostasis.