Intrinsic MyD88-Activation in B Cells Is Necessary for Development of Alloreactive Memory T Cells.

Abstract

B cells help alloreactive T cells to differentiate into memory T cells. Since B cells express TLRs that recognize endogenous ligands elicited during transplantation, we asked whether intrinsic MyD88-mediated activation of B cells is required for alloantibody production or T cell activation and differentiation to memory T cells. Methods: We made bone marrow chimeras lacking MyD88 only in B cells. Irradiated μMTCD45.1 mice were transplanted with bone marrow cells from μMTCD45.1 and wtCD45.2 (μMT+wt) or μMTCD45.1 and MyD88-/-CD45.2 (μMT+MyD88-/-). Differences in congenic background allowed us to confirm the reconstitution. MyD88 deficiency detected as CD45.2+ cells was restricted to B cells (98±0.5% of B cells). μMT+MyD88-/- (H2b) and μMT+wt (H2b) received Balb/c (H2d) skin transplants 12 weeks after reconstitution of bone marrow. Results: Rejection of skin allografts was comparable between μMT+MyD88-/- and μMT+wt chimeras (MST=18 & 17 days, respectively). μMT+MyD88-/- recipients showed 3-fold more alloreactive IFNγ+ CD4 and CD8 effector T cells when compared to μMT+wt mice at day 14 post-transplantation (p<0.05). Diminished regulatory T cells were also observed in μMT+MyD88-/- group. Alloantibodies of IgM, IgG1, and IgG2b were preserved while IgG2a alloantibodies were impaired in μMT+MyD88-/-chimeras. IL-10+ B cells were diminished (p<0.005) in μMT+MyD88-/- whereas IL-6 and TNFα in B cells and DCs were increased than in μMT+wt chimeras. CD8 memory precursor T cells (CD44hi CD62Llo CD127hi) in μMT+MyD88-/- showed predominantly Bcl2lo expression in both KLRG1lo memory precursor effector cell (MPEC) and KLRG1hi short-lived effector cell (SLEC) populations making them prone for increased contraction. Consistent with these findings, fewer alloreactive CD4 and CD8 IFNγ+ memory T cells were seen in μMT+MyD88-/- chimeras (p<0.005). This decreased number of IFNγ+ memory T cells were associated with impaired recall response in μMT+MyD88-/- measured as rejection of second set BALB/c skin grafts in presence of costimulation blockade when compared to μMT+wt chimeras (MST = 27 vs. 15 days, p < 0.005). Conclusions: (a) Absence of B cell MyD88 results in enhanced alloreactive T cell differentiation into into terminal effector T cells. (b) Intrinsic MyD88-dependent B cell functions are important for alloreactive T cell differentiation into long-lived memory T cells.