Mechanisms of Rejection of High Grade B cell Lymphoma in Mice

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.