Donor‐specific memory CD4 T cells provide help for alloantibody production in the absence of CD40/CD154 interactions and germinal center formation

Abstract

CD40 signaling is essential for immunoglobulin class switching in response to T‐dependent antigens. We investigated whether memory CD4 T cell help can be delivered in the absence of CD40/CD154 interactions.We used either polyclonal alloreactive memory CD4 T cells from C57BL/6 (B6, H‐2b) recipients of C3H (H‐2k) skin allografts or TCR transgenic Mar memory CD4 T cells reactive to male HYDby peptide. Wild type B6 or CD40−/− female mice injected with naïve or memory CD4 T cells received heterotopic heart transplants from male C3H donors, donor‐reactive serum alloAb titers were determined by flow cytometry.We observed that memory but not naïve CD4 T cells induced high titers of donor‐specific IgG Ab in wild type B6 recipients treated with anti‐CD154 mAb MR1. After adoptive transfer into CD40−/− mice, memory, but not naïve CD4 cells provided help to CD40−/− B cells to produce donor‐reactive IgG1, IgG2a, IgG2b and IgG3 as early as 1 week post transplant. Despite high titers of serum IgG, PNA and B220 staining revealed no germinal center formation in the spleens of CD40−/− recipients containing memory CD4 T cells. AlloAb production was not affected by the treatment of CD40−/− recipients with MR1 Ab or anti‐ICOS mAb, indicating that the memory CD4 T cell help to CD40−/− B cells was independent of CD154 engagement by an alternative ligand or ICOS/B7RP‐1 pathway.In summary, donor‐specific memory CD4 T cells can initiate alloAb response in CD40‐deficient heart allograft recipients. Thus, therapeutic interruption of CD40/CD154 signaling may fail to efficiently block de novo alloantibody production in T cell sensitized transplant patients.