Infiltration of CD8+ T cells into tumor cell clusters in triple-negative breast cancer

Abstract

Significance The infiltration of cytotoxic T cells into tumors is a critical factor in immunotherapy efficacy. We developed an algorithm to quantify infiltration level using whole-section immunohistofluorescence images from different patients. We observed consistent infiltration patterns of cytotoxic T cells (on the tumor cell cluster level) at the core and margin of each tumor. These spatial distributions enabled us to generate several hypotheses for mechanisms underlying the various infiltration levels. We used mathematical models to test which hypothetical mechanisms could recapitulate these patterns. One hypothesis, that of a fibrotic barrier, was shown to be unlikely. The other, involving a T cell chemorepellent, agrees with the data and can be tested in future experiments that directly measure the chemokine spatial patterns. Infiltration of CD8+ T lymphocytes into solid tumors is associated with good prognosis in various types of cancer, including triple-negative breast cancer (TNBC). However, the mechanisms underlying different infiltration levels are largely unknown. Here, we have characterized the spatial profile of CD8+ T cells around tumor cell clusters (tightly connected tumor cells) in the core and margin regions in TNBC patient samples. We found that in some patients, the CD8+ T cell density first decreases when moving in from the boundary of the tumor cell clusters and then rises again when approaching the center. To explain various infiltration profiles, we modeled the dynamics of T cell density via partial differential equations. We spatially modulated the diffusion/chemotactic coefficients of T cells (to mimic physical barriers) or introduced the localized secretion of a diffusing T cell chemorepellent. Combining the spatial-profile analysis and the modeling led to support for the second idea; i.e., there exists a possible chemorepellent inside tumor cell clusters, which prevents CD8+ T cells from infiltrating into tumor cell clusters. This conclusion was consistent with an investigation into the properties of collagen fibers which suggested that variations in desmoplastic elements does not limit infiltration of CD8+ T lymphocytes, as we did not observe significant correlations between the level of T cell infiltration and fiber properties. Our work provides evidence that CD8+ T cells can cross typical fibrotic barriers and thus their infiltration into tumor clusters is governed by other mechanisms possibly involving a local repellent.