Structured Abstract
INTRODUCTION
Cancer cells typically produce molecules (antigens) that are distinct from those made by normal cells and can be detected by the adaptive immune system. Most cancer immunotherapy strategies, including immune checkpoint blockade and cell therapies, leverage the ability of lymphocytes to directly kill tumor cells that express such specific antigenic targets and “present” them on their surface. Cancers escape elimination by limiting lymphocyte access to tumor cells or through the selective survival of initially rare variants that do not express or present the antigen. Complementary approaches that target the tumor stroma rather than the tumor cells themselves could help overcome these limitations.
RATIONALE
CD4+ T cells are central regulators of immune responses and tissue homeostasis. They exert pleiotropic functions by producing secreted and membrane-bound mediators that control the differentiation and function of other immune and nonimmune tissue components. CD4+ T cells often colonize human tumors, and pioneering clinical reports have demonstrated their therapeutic potential. However, consistent with their multifaceted activities, CD4+ T cells can promote or control tumor growth. CD4+ T cells recognize antigens, including tumor-specific antigens, that are presented by professional, noncancerous antigen-presenting cells with phagocytic activity. We reasoned that CD4+ T cells may exert indirect, stroma-mediated antitumor functions and set out to identify the underlying mechanisms.
RESULTS
We established murine experimental systems in which tumor cells, tumor-specific CD4+ T cells, and host tissues could each be genetically manipulated, allowing independent analysis of the function of each component.
Using several distinct models, we found that CD4+ T cells countered the growth of tumors expressing their target antigen. Tumor control did not require direct antigen presentation by tumor cells and did not involve additional lymphocyte subsets. Instead, it depended on the recruitment of monocyte-derived macrophages to the tumor, accompanied by their functional reprogramming toward inflammatory activity.
Using multiplex tissue imaging together with single-cell and tissue transcriptomics, we found that CD4+ T cells induced the formation of myeloid cell aggregates around intratumoral blood vessels that included inflammatory macrophages. These macrophages produced tumor necrosis factor (TNF), which was required for tumor control. TNF did not act directly on cancer cells but instead damaged intratumoral blood vessels, inducing the death of endothelial cells and pericytes, impairing tissue perfusion, and resulting in hypoxia and extensive tumor cell death. Unlike previously described CD4+ T cell antitumor mechanisms, activation of this CD4+ T cell–macrophage–TNF cascade required CD4+ T cell–derived interleukin-3 (IL-3) but not the prototypical macrophage-activating cytokine interferon γ. Analyses of human spatial transcriptomic datasets identified similar perivascular arrangements of CD4+ T cells, myeloid cells, vascular structures, and hypoxic tumor regions, suggesting that these findings in mice are relevant to human cancer.
CONCLUSION
These results identified a distinct mode of CD4+ T cell–mediated tumor control that induces inflammatory remodeling of the tumor stroma and disrupts tumor vasculature rather than directly killing cancer cells. This mechanism expands the known repertoire of CD4+ T cell antitumor functions and suggests therapeutic approaches that could complement current strategies focused on direct antigen receptor–mediated killing of tumor cells.
CD4+ T cells recognize tumor antigens near the tumor vasculature and promote the recruitment and inflammatory differentiation of macrophages around blood vessels.
T cell secretion of IL-3 in these perivascular clusters induces macrophages to secrete TNF that disrupts vascular integrity, leading to impaired perfusion, reduced oxygen and nutrient delivery, and subsequent tumor cell death. Ag, antigen; APC, antigen-presenting cell; TAM, tumor-associated macrophage.