Contrasting Effects of Post-Transplant Lymphopenia on Proliferation and Degranulation in Antigen-Specific CD8 Memory T Cells.

Lymphopenia following bone marrow transplant (BMT) can induce expansion of donor T cells which help contribute to reconstitution in recipients. We are interested in elucidating the responses of transplanted CD8 T memory (TM) to lymphopenic signals in the recipient splenic and marrow environments. To compare proliferative responses of marrow and spleen populating antigen-specific CD8 TM to lymphopenic signals, purified (95% pure) CFSE labeled B6 CD8 TM were co-transplanted with syngeneic or allogeneic TCD-BM to ablatively conditioned (9.0 Gy) syngeneic B6 recipients. Antigen-specific CD8 TM were obtianed following priming B6 mice against BALB.B antigens. An H60 tetramer (LTFNYRNL/H2-K^b) conjugated to PE was used to identify immunodominant H60-specific CD8⁺ cells which expressed a central memory (TM) phenotype (CD44⁺, Ly6C⁺). Three days post-transplant of these CD8 TM, recipient spleen and marrow cells were analyzed for CFSE dilution in gated H60⁺CD8⁺ T cells. Transferred H60⁺ CD8⁺ cells in the marrow compartment consistently showed greater (5 – 6 cell divisions) proliferative activity compared to those populating the spleen (1 – 2 divisions) in recipients of syngeneic (i.e. homeostatic) as well as allogeneic TCD-BM. Thus, the marrow compartment apparently provided greater support for the early homeostatic proliferation of these MiHA-specific CD8 TM. Furthermore, the presence of antigen in B6 recipients of allogeneic BM did not significantly enhance the proliferation of transferred CD8⁺ TM, indicating that homeostatic signals induced greater overall proliferation than antigen activation in the simultaneous presence of both signals early post-transplant. To examine for non-proliferative functional activity in transplanted antigen-specific CD8 TM, degranulation was assessed by surface expression of LAMP-1 (CD8 107a) which was analyzed 3 days after the transfer of purified splenic CD8 T cells from B6_BALB.B into lethally irradiated (9.0 Gy) syngeneic B6 mice. Analysis of recipient spleeen and marrow showed that transferred antigen-specific in these tissues markedly upregulated surface LAMP-1 expression. Interestingly, there was greater frequency (~90%) of CD8⁺ TM in the spleen expressing LAMP-1 than in the bone marrow (56%). To determine if this degranulation response was limited to transplanted TM, a homogeneous population of naive T cells (TN) from OT-1 RAG^−/− TCR tg mice was prepared and transplanted in an identical manner. LAMP-1 expression was marginally detectable in these cells. In total, these results suggest that transplanted CD8 TM and TN respond differently in the homeostatic environment post-transplant. Moreover the observations may suggest that CD8 TM can exhibit effector-like activity in the lymphopenic milieu immediately post-transplant. These findings have implications for the relative contribution of hematopoietic compartments during immune reconstitution after HCT.