The goal of allogeneic hematopoietic stem cell transplantation (allo-HSCT) is to achieve graft-versus-leukemia (GVL) effect to eliminate residual tumor cells in host [left after irradiation]. Since most allo-HSCs are derived from HLA-compatible donors, GVL reaction is mediated by donor T cells reacting against host minor histocompatibility antigens (miHAg; self or tumor-specific Ags). However, even reaction to self-miHAg could still induce graft-versus-host disease (GVHD) because self-miHAgs are broadly expressed in different tissues. Interestingly, GVHD preferentially affects peripheral tissues such as liver or gut. But why?
A new [and very good] paper in journal Immunity may have an answer for this question. In this study, the authors found that selective expression of PD-1 ligands, PD-L1 and PD-L2, in allo-HSCT recipients' lymphoid tissues [but not in peripheral tissues] provide a "cover" against cytotoxic activity exerted by donor CD8 T cells.
To mimic allo-HSCTs, the authors have used female-to-male BM stem cell transplantation model. In this model, self-miHAg is represented by male antigen, HY. To induce GVHD, female BM cells were transplanted alongside with HY-specific transgenic MataHari CD8 T cells. As expected, only male recipients of female HSCs + MataHari CD8 T cells developed GVHD (in liver, gut and skin tissue).
To address the question why only those peripheral tissues were affected by GVHD, the authors examined the hypothesis that HY specific CD8 T cell cytotoxic activity were differentially affected by different tissues. Indeed, co-transfer of labeled male and female targets revealed that recipients of allo-HSCs + MataHari CD8 T cells showed selective reduction of cytotoxic activity against male targets in lymphoid tissues (but not in liver).
This observation was supported by the fact that in contrast to peripheral tissues, MataHari CD8 T cells obtained from lymphoid tissues expressed low level of granzyme B (molecule involved in cytotoxic activity).
To understand why it is the case, the authors examined expression of inhibitory molecules on CD8 T cells. This revealed that while PD-1 on CD8 T cells were similarly expressed irrespective of tissue origin, its ligands, PD-L1 and PD-L2 were selectively up-regulated in lymphoid tissues.
The role of PD-L1 and PD-L2 in inhibition of CD8 T cell cytotoxicity in lymphoid tissues were confirmed in experiment with anti-PD1 antibody.
Finally, using anti-PD-1 antibody injection, the authors showed that B cell leukemia cells that were hiding in lymphoid tissues of allo-HSCs male recipients could be now eliminated by MataHari CD8 T cells.
In summary, this study showed that (a) during GVHD donor CD8 T cell activity is differentially regulated by different tissues based on availability of inhibitory PD-1 signaling and that (b) this is exploited by leukemia cells to hide in lymphoid tissues but it could be overcome by anti-PD1 antibody injection.