CD8+ T cell concentration determines their efficiency in killing cognate antigen expressing syngeneic mammalian cells in vitro and in mouse tissues

We describe a quantitative model for assessing the cytolytic activity of antigen-specific CD8(+) T cells in vitro and in vivo in which the concentration of antigen-specific CD8(+) T cells determines the efficiency with which these cells kill cognate antigen-expressing melanoma cells in packed cell pellets, in three-dimensional collagen-fibrin gels in vitro, and in established melanomas in vivo. In combination with a clonogenic assay for melanoma cells, collagen-fibrin gels are 4,500-5,500-fold more sensitive than the packed cell pellet-type assays generally used to measure CD8(+) T cell cytolytic activity. An equation previously used to describe neutrophil bactericidal activity in vitro and in vivo also describes antigen-specific CD8(+) T cell-mediated cytolysis of cognate antigen-expressing melanoma cells in collagen-fibrin gels in vitro and in transplanted tumors in vivo. We have used this equation to calculate the critical concentration of antigen-specific CD8(+) T cells, which is the concentration of these cells required to hold constant the concentration of a growing population of cognate antigen-expressing melanoma cells. It is similar to 3.5 x 10(5)/ml collagen-fibrin gel in vitro and similar to 3 x 10(6)/ml or /g melanoma for previously published studies of ex vivo-activated adoptively transferred tumor antigen-specific CD8(+) T cell killing of cognate antigen-expressing melanoma cells in established tumors in vivo. The antigen-specific CD8(+) T cell concentration required to kill 100% of 2 x 10(7)/ml cognate antigen-expressing melanoma cells in collagen fibrin gels is >= 10(7)/ml of gel.