Cooperative action of CD8 T lymphocytes and natural killer cells controls tumour growth under conditions of restricted T‐cell receptor diversity

In mice expressing a transgenic T-cell receptor (TCR; TCRP1A) of DBA/2 origin with reactivity towards a cancer-germline antigen P1A, the number of TCRP1A CD8⁺ T cells in lymphoid organs is lower in DBA/2 than in B10.D2 or B10.D2(× DBA/2)F₁ mice. This reduction results from haemopoietic cell autonomous differences in the differentiation of the major histocompatibility complex class I-restricted TCRP1A thymocytes controlled by DBA/2 versus B10.D2-encoded genes. We report here that the lower number of TCRP1A CD8⁺ T cells in DBA/2 mice correlated with their poor resistance to P1A-expressing mastocytoma solid tumours. Functional potency of CD8⁺ cytolytic T lymphocytes (CTL) from the above strains was not compromised, but their number after expansion appeared to be influenced by their genetic background. Intriguingly, non-transgenic DBA/2 mice resisted P1A⁺ tumours more efficiently despite poor representation of P1A-specific CTL. This was partly the result of their more heterogeneous TCR repertoire, including reactivity to non-P1A tumour antigens because mice that had rejected a P1A⁺ tumour became resistant to a P1A⁻ variant of the tumour. Such ‘cross-resistance’ did not develop in the TCRP1A transgenic mice. Nonetheless, reconstitution of RAGº^/º mice with TCRP1A CD8⁺ T cells, with or without CD4⁺ T cells, or exclusive representation of TCRP1A CD8⁺ T cells in RAGº^/º TCRP1A transgenic mice efficiently resisted the growth of P1A-expressing tumours. Natural killer cells present at a higher number in RAGº^/º mice also contributed to tumour resistance, in part through an NKG2D-dependent mechanism. Hence, in the absence of a polyclonal T-cell repertoire, precursor frequencies of natural killer cells and tumour-specific CTL affect tumour resistance.