Identification of CD8+ T cell-stimulating shared antigens that are uncovered in CT26 vaccinated mice in the absence of CD25+ regulatory T cells

Abstract

There is evidence suggesting CD4+CD25+ regulatory T cells may impede generation of effective immunity against tumours as they were involved in maintaining tolerance to self antigen.  CT26 tumour cells are rejected in BALB/c mice that have been in vivo depleted of CD25 regulatory T cells.  In addition, the activated tumour immunity is also effective against tumours of different histological origins, such as A20, BCL1 and RENCA, suggesting that shared murine tumour-associated antigens were unveiled in the absence of CD25+ regulatory T cells.  The characterization of such shared antigens would lead to increase in understanding the roles of CD4+CD25+ regulatory T cells in tumour immunity.

The identification of unknown tumour antigens essentially depends on the use of CD8+ T cells as probes for the screening of recombinant tumour cells cDNA libraries.  Here, we generated from immunized BALB/c mice two tumour-reactive CD8+ T cell lines, from which, several CTL clones were isolated.  These CTL clones were shown to be cross-reactive to several histologically different tumour cells with different H-2^d restriction.  We also used somatic hybridization to establish a line of functional hybridoma cells generated by fusion of CTL clone 3 with BWZ36/CD8α cells that carry a construct of NFAT-LacZ gene.

Pools of cDNA from a CT26 tumour-derived cDNA library were transiently transfected into Cos-7 cells to establish an expression cloning system.  Taking advantage of TNF/WEHI assay for CTL and CPRG assay for hybridoma cells, the CT26 cDNA libraries were screened for the identification of the CD8+ T cell-reactive tumour antigens.

Here, an endogenous murine leukemia virus (MuLV) pol/env antigen was identified as a shared tumour antigen operating in the cross-protection in CT26-vaccinated Treg depleted BALB/c mice.

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