Mechanisms of Rejection of High Grade B cell Lymphoma in Mice

Abstract

The incidence of high grade B cell lymphoma in western countries has increased over the last decades. Improvement of conventional chemotherapy regimens has significantly contributed to prolonged 5-year survival rates which currently reach around 60%. However, relapse after conventional chemotherapy is an important challenge, especially in high grade B cell lymphomas. The potential benefit of immunological approaches for the elimination of such lymphomas still remains unclear. In this study, we attempted to address whether the forced expression of foreign antigens in a tumor of B cell origin leads to immune recognition and elimination of the tumor and to assess the potential role of IFN-gamma (IFN-g) in tumor rejection. To this end, we used a transgenic mouse lymphoma model, where the human proto-oncogene c-myc (a foreign antigen for the mouse host) is under the control of regulatory elements of the immunoglobulin lambda locus, thereby recapitulating the important features of a t(8;22) translocation as found in human Burkitt’s lymphoma. From these spontaneously developing tumors, lymphoma cell lines were established that either express (line 291) or are deficient (line 9) in Stat1- a key signaling molecule in the response to interferons. We found that the expression of foreign antigens such as chicken ovalbumin (OVA) and green fluorescent protein (GFP) in Stat1-competent 291 cells led to immune responses that delayed tumor progression and improved survival of wild-type animals. Consistent with this, loss of foreign antigen inevitably led to accelerated tumor progression upon transfer into immunocompetent wild-type mice. Transfer of immunogenic 291-OVA-GFP lymphoma cells led to increased tumor progression without loss of foreign antigen upon transfer into IFN-γ-/- and Stat1-/- mice indicating that no selection of antigen loss-variants occurred in these mice. The rejection of 291-OVA-GFP cells in wild-type mice was at least in part mediated by CD8+ T cells as measured by enrichment of the OVA antigen-derived MHC class I-restricted SIINFEKL epitope-specific cells in wild-type recipients.. Interestingly, Stat1-deficient lymphoma cells (9-GFP and 9-OVA-GFP) were rejected by immunocompetent UBQ-GFP transgenic wild-type C57BL/6 mice irrespectively of the presence of a foreign antigen, indicating the existence of immunosurveillance against these Stat1-deficient lymphomas. To evaluate the key players behind lymphoma rejection, we transferred 9-GFP cells into IFN-γ-/- and Stat1-/- recipients. This led to enhanced tumor growth indicating that endogenous IFN-γ production and Stat1 signaling are critical for tumor rejection. To gain an insight into the mechanistic aspects of innate immunosurveillance against the Stat1-competent and Stat1-deficient lymphomas, NK cell functionality was evaluated. We found that NK cells could efficiently lyse both Stat1-competent and Stat1-deficient lymphoma cell lines in vitro. Treatment with IFN-γ increased the susceptibility of Stat1-deficient lymphoma cells to NK cell killing, but decreased that of Stat1-competent cells, presumably by upregulating MHC class I expression. The results of this work show that host IFN-γ and Stat1 signaling are important for tumor clearance, and that paradoxically, the absence of Stat1 within the lymphoma is required for rejection