Protection from lethal graft-vs.-host disease by donor stem cell repopulation

Subclinical GvHD in non-irradiated F1 hybrids: severe lymphoid-tissue GvHD causing prolonged immune dysfunction

GvHD is an important complication of allogeneic hematopoietic SCT. Parent-in-F1 models are frequently used to study GvHD immunobiology; the characteristics of parent-in-F1 GvHD vary between strain combinations and induction protocols. Here, we observed that a high-dose challenge of non-irradiated B6DBA2F1 and B6SJLF1 recipients with C57BL/6 splenocytes left the majority of recipients clinically healthy, while inducing progressive high-grade donor T-cell chimerism. We investigated this previously undescribed pattern of parent-in-F1 T-cell alloreactivity and studied the effect of serial parental splenocyte infusions on epithelial and lymphohematopoietic tissues. The majority of recipients of 4 weekly splenocyte infusions showed long-term survival with gradual establishment of high-grade donor chimerism and without any signs of epithelial-tissue GvHD. A minority of recipients showed BM failure type of GvHD and, respectively, graft rejection. Moreover, long-term F1 chimeras showed protracted pancytopenia, and in peripheral lymphoid tissues severe lymphopenia and near-complete eradication of APCs and dysfunction in antigen-presenting capacity in remaining APC. Hematopoiesis and lymphoid tissue composition recovered only after multilineage donor chimerism had established. In conclusion, we report on a novel type of parent-in-F1 hybrid GvHD, where a cumulative high dose of C57BL/6 parental splenocytes in non-irradiated F1 mice induces subclinical but severe hematolymphoid-tissue GvHD, causing prolonged immuno-incompetence.

Fas expression on antigen-specific T cells has costimulatory, helper, and down-regulatory functions in vivo for cytotoxic T cell responses but not for T cell-dependent B cell responses

Fas-mediated apoptosis is an important contributor to contraction of Ag-driven T cell responses acting only on activated Ag-specific T cells. The effects of targeted Fas deletion on selected cell populations are well described however little is known regarding the consequences of Fas deletion on only activated Ag-specific T cells. We addressed this question using the parent-into-F(1) (P-->F(1)) model of acute or chronic (lupus-like) graft-vs-host disease (GVHD) as a model of either a CTL-mediated or T-dependent B cell-mediated response, respectively. By transferring Fas-deficient lpr donor T cells into Fas-intact F(1) hosts, the in vivo role of Ag-specific T cell Fas can be determined. Our results demonstrate a novel dichotomy of Ag-specific T cell Fas function in that: 1) Fas expression on Ag-activated T cells has costimulatory, helper, and down-regulatory roles in vivo and 2) these roles were observed only in a CTL response (acute GVHD) and not in a T-dependent B cell response (chronic GVHD). Specifically, CD4 T cell Fas expression is important for optimal CD4 initial expansion and absolutely required for help for CD8 effector CTL. Donor CD8 T cell Fas expression played an important but not exclusive role in apoptosis and down-regulation. By contrast, CD4 Fas expression played no detectable role in modulating chronic GVHD induction or disease expression. These results demonstrate a novel role for Ag-specific T cell Fas expression in in vivo CTL responses and support a review of the paradigm by which Fas deficiency accelerates lupus in MRL/lpr lupus-prone mice.