Donor T Cells from B Cell Deficient Mice Inhibit B Cell Development in Normal Recipients after Hematopoietic Cell Transplantation.
Authors: Mueller Antonia M.S. , Allen Jessica A. , Miklos David , Shizuru Judith A. 
Published: October 2019

Abstract
Allogeneic hematopoietic cell transplant (HCT) recipients often exhibit B cell (BC) lymphopenia due, in part, to graft-versus-host-disease (GVHD). Here, we studied the impact of donor T cells (TC) on BC deficiency post minor antigen-mismatched HCT. Following lethal irradiation, BALB.B mice were given FACS purified hematopoietic stem cells (HSC: cKIT+Thy1.1loLin-Sca-1+) alone, with whole splenocytes (SP), CD4 or CD8 TC from minor antigen-mismatched C57BL/6 (B6) mice. Chimerism analyses were performed on day (d) 30, 60, and 90. When pure HSC were transplanted, BCs reconstituted promptly (median 33% of lymphocytes [d30]; 61% [d60]; 74% [d90]), whereas TC engraftment was retarded and did not achieve full donor chimerism. Addition of SP or CD4 TCs, or to a lesser degree CD8 TCs, delayed BC reconstitution, with extremely low percentages of BCs beyond d60. This BC suppression correlated with the degree of acute GVHD, and BC numbers increased with recovery from GVHD. Additionally, this BC suppression was in stark contrast to TC development, with TC transfer resulting in early conversion to full donor chimerism. To test if previous events in the donor sensitize TCs against BC features (e.g. minor antigens), thereby promoting anti-BC cytotoxicity post-HCT, TCs from B6 muMT mice were co-transplanted with HSC. muMT mice are devoid of mature BCs because they lack the mu chain; consequently, their TCs were not exposed to BCs prior to transfer. Remarkably, BC engraftment was completely prevented through d90. TCs regenerated faster, but the vast majority originated from spleen and not HSC. To differentiate this lack of BC engraftment from GVHD-associated, alloreactive BC lymphopenia, syngenic B6 recipients were used. Again, initially complete blockade of BC engraftment was observed, although this suppression was overcome earlier post-HCT as compared to the minor-mismatched pair (median % BC d60: ’HSC only’ recipients 52%; +CD4 17%; +CD8 48%). To clarify if this phenomenon was a purely cytotoxic reaction of muMT TC against BCs, we used WT B6 HSC +/− SP as donors and lethally or sublethally irradiated muMT mice as recipients. All groups, including sublethally irradiated animals, where host muMT TC were still present, engrafted BCs making a direct anti-BC cytotoxicity unlikely as the sole cause of the BC inhibition. FACS analysis of bone marrow was used to assess the developmental stages of BCs (Hardy fractions (Fr.) A-F) and revealed GVHD recipients with peripheral B lymphopenia have a shift of B220+ cells from more mature Fr. D-F to immature Fr. A-C stages and a lower proportion of IgM expressing BC. Recipients of the muMT TCs showed, in addition to a shift to more immature stages, a clear block in BC development with an absent switch to the expression of IgM (stage D to E)(Fig. 1). In conclusion, muMT TCs are capable of blocking BC maturation when transferred into WT mice, suggesting defective TC activity in muMT animals necessary for the co-development of both BCs and TCs. Furthermore, this study provides evidence that mature TCs are capable of interfering with BC regeneration post-HCT. Hence, our HCT combinations using WT and muMT B6 mice provide a powerful tool to study the role of TC function in the process of donor BC development post-HCT.