Allo-Antigen Expression on Both APCS and Tumor Is Required To Elicit an Effective GVL Response after Experimental Allogeneic BMT.

The interactions between donor T cells, antigen presenting cells (APCs) and malignant target cells are not well defined in graft-versus-leukemia/tumor (GVL) effect. We used a series of bone marrow chimeras to address these questions. C57BL/6 (B6-H2^b) and B6.C-H-2^bm1 (bm1) mice were conditioned with 11 Gy and injected with T cell depleted bone marrow from syngeneic B6 Ly5.1 or allogeneic bm1 donors that differ from each other at a single class I antigen. Complete donor chimerism was confirmed by flow cytometry. Hematopoietic derived APCs and the target tissues are syngeneic in the [B6→B6] chimeras and allogeneic in the [bm1→B6] and [B6→bm1] chimeras. Chimeras were re-irradiated 3 months later and injected with MBL (H-2^b) tumor cells along with TCD BM and CD8+ cells from B6 or bm1 donors. Survival was monitored and histologic presence of tumor determined at necropsy (see table). When donor T cells, host APCs and tumor were syngeneic to each other (group 1), 100% of animals died from tumor. When the APCs and tumor were syngeneic to each other but allogeneic to donor T cells (group 2), 10/ 13 animals died from GVHD and only 1/ 13 died from tumor demonstrating both GVHD and GVL. Group 3 was identical to group 2, but animals were injected with 100μg of TNFR-Fc and 200μg of αIL-1R on days −2, −1, 0 and then on alternate days up to day 35, resulting in significantly reduced GVHD but preserved GVL (0/7 tumor deaths). When APCs and tumor were allogeneic to donor T cells but target tissues were syngeneic (bm1→[B6→bm1], group 4), cytokine blockade again significantly reduced GVHD and preserved GVL (7/ 9 tumor free survivors). When no GVH reaction was elicited because APCs were syngeneic to the donor T cells (bm1→[bm1→B6], group 5), the animals lived longer than the syngeneic controls, but GVL was lost and all animals died of tumor even though the tumor was allogeneic to the donor T cells. By contrast, when donor T cells were allogeneic to APCs but syngeneic to the tumor (group 6), all animals died from tumor, showing that allo-antigen expression on tumor is an absolute requirement for GVL. We confirmed the functional requirement of APCs for GVL effect in the C3H.SW → [B6→B6] model of CD8⁺ mediated GVHD to minor H antigens. When APCs were functional (group 7), no animal died of tumor. By contrast when APCs were incapable of allo-antigen presentation because of lack of MHC class I, 17/ 21 animals died of tumor. Similar results were also obtained in a second tumor, C6VL (H2^b), model. Together the results of these model systems demonstrate: 1) allogeneic APCs are required for CD8⁺ mediated GVL (2) allogeneic APCs do not elicit GVL unless the tumor also expresses allo-antigen(s) (3) blockade of pro-inflammatory cytokines does not inhibit GVL in these models. Our data thus provide for a new perspective on the requirements for donor T cells, APCs and tumor allo-antigens and have important implications for improving the immunotherapeutic potential of allogeneic BMT.

* Groups 1 v 2, 3, 4 and 5, P < 0.03

**Groups 2 v 6, and 7 v 8, P < 0.001