Allogeneic Rejection of High Grade Lymphoma: Evidence for Immune Escape in a Haploidentical Murine Model.

Allogeneic stem cell transplantation for high grade lymphoma typically suffers from high transplant related mortality and minimal success in achieving long term complete remission. The immunological aspects behind rejection of high grade lymphoma are poorly understood, partly due to the lack of animal models. Using a transgenic mouse lymphoma model, where the proto-oncogene c-myc is driven by parts of the immunoglobulin lambda locus representing a t(8;22) translocation as found in Burkitt’s lymphoma, we developed a haploidentical transplantation model. c-myc-lambda transgenic C57/BL6 mice were crossbred with DBA mice, giving rise to a B6D2F1 generation which develops high grade lymphoma spontaneously within the first 6 months post birth. Primary F1-lymphoma cell lines displayed low MHC class I and class II expression compared to wild type B-cells. When transferred into immune-competent, healthy C57/BL6 parental mice, F1-lymphoma cells were rejected even at high doses of 50 Mio. cells, whereas the syngeneic transfer of 10,000 cells into B6D2F1 mice resulted in 100% mortality due to lymphoma growth. Immunosuppression of C57/BL6 mice by lethal total body irradiation and transplantation of T-cell depleted bone marrow allowed F1-lymphoma to grow when animals were challenged at day +1 after bone marrow transplantation with 1 Mio. cells. Addback of 2.5 or 10 Mio. parental T-cells together with F1-lymphoma on day +1 completely prevented lymphoma growth, and resulted in long term survival for more than 60 days. When T-cell addback was delayed until day +3 or +6 after lymphoma challenge, animals died due to lymphoma progression 7–8 days later. By using “green” T-cells from GFP transgenic C57/BL6 mice we could show that spleens 7 days after T-cell addback contained only 0.5% (15% of total CD3+ cells) GFP positive T-cells, whereas up to 15% (55%–75% of total CD3+ cells) of total splenocytes were GFP positive in long term survivors that rejected lymphoma. Similar results were obtained when T-cells were labelled with CFDA. This indicates that T-cell expansion is associated with lymphoma rejection; animals that receive T-cells after lymphoma has been established do not show any T-cell expansion, even when lymphoma cells display a haplo-mismatch and mature T-cells are co-localized within the spleen. T-Cell expansion is suppressed when lymphoma cells are present in splenic tissue, indicating efficient immune escape. Similar results were observed when bone marrow and T-cells from C57BL/6 donors were transplanted into irradiated B6D2F1 mice and 1 Mio. F1-lymphoma cells were injected on day +1 post transplantation. Despite the presence of GvHD when low number of T-cells (0.5–2.0 Mio.) were used lymphoma growth was measurable and animals eventually died due to disease progression. These results point to the crucial role of pretransplant lymphoma burden for the success of allogeneic stem cell transplantation in high grade lymphoma.