Preprint Club
A cross-institutional Journal Club Initiative

Keywords

• CD4+ T helper 17 (Th17) cells

• Tumor Immunology

• Mucosal Immunity

• Intestinal Microbiota Translocation

• Immune Checkpoint Blockade

Main Findings

ICB has been shown to induce intestinal antigen and bacterial translocation into tumors and lymph nodes, defining an improved response with certain bacterial presence. Antigen stimulation and presentation to T cells is evident in the form of live and dead bacteria, excreted metabolites, and bacterial cell components to induce anti-tumor immune responses. Mechanisms remain unclear on how this tumor growth is controlled from the perspective of T cells with receptor cross-reactivity to tumor antigens. Using a model with an engineered, SFB-antigen expressing, melanoma TME, the authors show how ICB response is improved when the murine microbiome is colonized with SFB. The improved response is characterized as a lamina propria stimulated, Th17 transitioning to effector Th1 mechanism, in the TME.

Anti-PD-1 effectively inhibits the growth of implanted SFB antigen-expressing melanoma only if mice are colonized with SFB. 

The authors used peptide-MHCII tetramers and T cell antigen receptor transgenic mice to adequately model a T cell specific response. Tumor growth in mice with a TME expressing SFB antigen and a colonized SFB microbiome demonstrated a significantly improved control of tumor growth when administered ICT.

Intestinal microbiota-educated ex-Th17 cells control tumor growth by producing high levels of the pro-inflammatory cytokines IFN-g and TNF-a, and promote effector functions of CD8+ tumor infiltrating cytotoxic lymphocytes. 

Exploration of CD4+ effector T cell functions in tumors of SFB+ and SFB- mice treated with anti-PD-1 was implemented as the main investigational strategy defending their central hypothesis. With SFB colonization and ICT, CD8+ T cell functions were enhanced with significant increases in IFNy, TNFa, and granzyme B in the TME. Additionally, they demonstrated an enrichment of CD4+ TILs in these same SFB+ mice treated with ICT. This phenotype was characterized by IFNy and TNFa expression and tetramer positivity of CD4+ TIL stimulation ex vivo.

Identified tumor-associated SFB-specific Th1-like cells derived from the homeostatic Th17 cells induced by SFB colonization in the small intestine lamina propria. 

Single-cell TCR sequencing identified a clonal relationship between CD4+ Th17 cells in the lamina propria and their transitioned, tumor associated, Th1 effector like cells in the TME. This was only evident in SFB colonized mice reinforcing the mechanism that these effector T cells develop their plasticity in an intestinal-microbiota dependent manner.

Limitations

• This investigation primarily focuses on lamina propria stimulated ex-Th17 – Th1 cells and their translocation to the TME. Only one endogenous microbiota species was used in this study and the colonization phenotype was always “forced” with an antibiotic depletion of the natural microbiome. This is not ideal for a recapitulation of the natural intestinal microenvironment.

• Endogenous bacterial translocation is not investigated. Using a model where antigen is expressed in the TME is an original idea, but it is not translatable if there isn’t a proven mechanism of that specific bacteria/antigen is entering the lymphatics and translocating to the tumor.

• Mesenteric lymphatics are not well investigated. Mesenteric lymph nodes are not characterized for bacterial translocation or ex-Th1 – Th17 cells.

• Clinical translational value is limited due to bacterial translocation not being investigated.

Significance/Novelty

This study delineates a novel mechanism in which ICT effectively inhibits the growth of implanted SFB antigen-expressing melanoma in a murine model where mice are colonized with the same endogenous strain. The authors build on current research showing the significance of cross-reactivity between microbial and tumor associated antigens in the TME. There is little understanding of how the gut microbiota can be utilized to enhance immune control of distal tumors. This study makes a big leap into the mechanism of how intestinal microbes control anti-tumor immunity.

Recommendations

This study demonstrates a unique and novel mechanism in a field that is yearning for mechanistic insight. Focus on antigenic mimicry and understanding of lamina propria derived CD4+ T helper cells is essential in understanding T cells with receptor cross-reactivity for tumor antigens. Validation is needed on the more natural state of the endogenous microbiome. Is translocation of SFB occurring into the mesenteric lymphatics? Are whole/live SFB making into the mesenteric lymph nodes, TDLN, and tumor at baseline? How does this phenotype change on administration of ICB and tumor implantation together and separately?

Credit

Reviewed by Samuel Andrewes as part of a cross-institutional journal club between the Icahn School of Medicine at Mount Sinai, the University of Oxford, the Karolinska Institute, the University of Toronto and MD Anderson Cancer Center.

The author declares no conflict of interests in relation to their involvement in the review.