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

Abstract

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 ∼3.5 × 105/ml collagen-fibrin gel in vitro and ∼3 × 106/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 × 107/ml cognate antigen-expressing melanoma cells in collagen fibrin gels is ≥107/ml of gel.

Tumors Suppress In Situ Proliferation of Cytotoxic T Cells by Promoting Differentiation of Gr-1+ Conventional Dendritic Cells through IL-6

“…Whether this mechanism participates in Gr-1 + cDC differentiation remains to be defined. The success of CTL therapy for cancer relies on achieving sufficient numbers of functional CTLs in the tumor (43,44). Our study indicates that transferred CTLs undergo multiple rounds of cell division in tumors, which was tumor Ag dependent.…”

Trafficking of T Cells into Tumors

“…The efficiency of adoptively transferred T cells infiltrating the tumor site and the persistence of these cells have been found to correlate well with clinical responses and outcomes in patients (44). In mouse studies, it has been confirmed that the concentration of adoptively transferred CD8 þ T cells within the tumor microenvironment was an important parameter, for whether these cells could effectively kill established cognate antigen-expressing tumors in vivo (45). However, of the large number of ex vivo expanded T cells, only a small fraction of these transferred T cells eventually reach the tumor tissue in both humans (44,46) and mice (18,47).…”

Combining Radiotherapy and Cancer Immunotherapy: A Paradigm Shift

“…Although radiation can promote homing and extravasation of effector T cells at the tumor site and induce the expression of molecules that enhance the recognition of the tumor by T cells (30,32,37,40,42,48), these steps are generally insufficient, thus explaining, in part, the rarity of abscopal effects. A critical concentration of fully functional T cells primed against the tumor is required to achieve immune-mediated tumor rejection in experimental tumor models (78,79) (80,81). In addition, myeloid cells populate the tumor microenvironment and promote tumor growth by suppressing T-cell functions.…”