Stromal and Immune Cell Dynamics in Tumor Associated Tertiary Lymphoid Structures and Anti-Tumor Immune Responses

Abstract

Tertiary lymphoid structures (TLS) are ectopic lymphoid organs that have been observed in chronic inflammatory conditions including cancer, where they are thought to exert a positive effect on prognosis. Both immune and non-immune cells participate in the genesis of TLS by establishing complex cross-talks requiring both soluble factors and cell-to-cell contact. Several immune cell types, including T follicular helper cells (Tfh), regulatory T cells (Tregs), and myeloid cells, may accumulate in TLS, possibly promoting or inhibiting their development. In this manuscript, we propose to review the available evidence regarding specific aspects of the TLS formation in solid cancers, including 1) the role of stromal cell composition and architecture in the recruitment of specific immune subpopulations and the formation of immune cell aggregates; 2) the contribution of the myeloid compartment (macrophages and neutrophils) to the development of antibody responses and the TLS formation; 3) the immunological and metabolic mechanisms dictating recruitment, expansion and plasticity of Tregs into T follicular regulatory cells, which are potentially sensitive to immunotherapeutic strategies directed to costimulatory receptors or checkpoint molecules.

Keywords: TLS; Tfh; Treg; neutrophils; tumor stroma.

FIGURE 1

Inductive sites of tumor-specific immune responses. Tumor cells express tumor associated antigens (TAAs) sampled by dendritic cells that migrate to tumor draining lymph nodes where adaptive immune responses are triggered. Tumors also feature tertiary lymphoid structures (TLS) where tumor specific T and B cells can be primed. In both cases, activated tumor-specific CD4⁺ and CD8⁺ T cells migrate within the tumor mass and attack transformed cells, ultimately inducing apoptotic cell death. Also plasma cells participate in antitumor immunity by producing tumor specific antibodies. aCD4, activated CD4 T cell; aCD8, activated CD8 T cell; nCD4, naive CD4 T cell; nCD8, naive CD8 T cell; PC, plasmacell; cDC, conventional dendritic cell; FDC, follicular dendritic cell. Created with BioRender.com.

FIGURE 2

Stromal and immune cells crosstalk in the generation and functions of tumor associated TLS. (A) Stromal components of tumor masses (fibroblasts, endothelial and myeloid cells) are the source of proinflammatory cytokines (e.g., IL-8) and chemokines (e.g., CXCL13) that drive recruitment and activation of lymphocytes and other myeloid cells (e.g., neutrophils) at the tumor site that generate organized lymphoid structures (B) Tumor associated TLS feature multiple interactions between lymphoid and myeloid cells for induction of tumor specific immune responses. Dendritic cells (DC) and B cells can present TAA on MHC class I and class II molecules to CD8⁺ and CD4⁺ T cells expressing the cognate T cell receptor (TCR) and, concomitantly, provide costimulatory signals by CD40⁻CD40L axis. Upon antigen recognition, CD8⁺ T cells differentiate into cytotoxic cells that kill tumor targets by perforin (PFN) and Granzyme B (GzmB). CD4⁺ T cells can differentiate into several T helper subsets, including T helper 1 (Th1) and T follicular helper (Tfh) that sustain, respectively, cytotoxic and B cell responses through the production of specific cytokines (IFNγ, IL-15, IL-21). Activated B cells differentiate into antibody secreting plasma cells. TAA-specific CD4⁺ T cells include regulatory T cells that, once activated, counteract effector CD4⁺ and CD8⁺ T cells. A specialized subset of Treg with a follicular phenotype (Tfr) inhibit B cell responses. Created with BioRender.com.