REDUCTION OF ALLOANTIBODY RESPONSE TO CLASS-I MAJOR HISTOCOMPATIBILITY COMPLEX BY TARGETING SYNTHETIC ALLOPEPTIDES FOR PRESENTATION BY B-CELLS

Background. PVG.RT1(u) develop a strong CD4 T cell-dependent alloantib ody response to class I major histocompatibility complex (MHG) A(a) an tigen, during which CD4 T helper cells recognize and respond to A(a)-d erived peptides presented by recipient class II MHC (indirect alloreco gnition). On the basis of evidence that CD4 T cells that encounter ant igen presented by resting B cells become tolerant, we have targeted sy nthetic A(a)-derived allopeptides for in vivo presentation to class I MMC-disparate CD4 T cells by resting recipient B cells. Methods. PVG.R T1(u) rats were treated with two peptides, P1 and P2, corresponding to the alpha-helical regions of A(a) (residues 57-80 and 143-163), which were conjugated via san N-terminal cysteine residue to monovalent Fab fragments of OX60 monoclonal antibody, which labels membrane IgD-posi tive B cells. Results. RT1(u) rats primed with free (nonconjugated) P1 or P2 emulsified in complete Freund's adjuvant produced strong peptid e-specific antibody responses and a heightened anti-A(a) antibody resp onse to an A(a)-disparate PVG.R8 heart graft, confirming that each pep tide encompasses one or more major T cell determinant for B cell help. Pretreatment of PVG.RT1(u) rats with a mixture of OX60-Fab-P1/P2 conj ugates markedly reduced their ability to mount an A(a) antibody respon se when challenged with either A(a)-disparate blood transfusion or an A(a)-disparate heart graft, although PVG.R8 heart graft survival was n ot prolonged. Conclusions. In this report, we show that synthetic A(a) -derived allopeptides sire able, when targeted for in vivo presentatio n to CD4 T cells by resting B cells, to impair the ability of RT1(u) r ats to mount an antibody response to A(a) antigen. All subclasses of I gG anti-A(a) alloantibody were profoundly reduced, suggesting that the responsible mechanism is more likely to be CD4 T helper cell unrespon siveness rather than Th1/Th2 T cell polarization.