A TLR4 agonist improves immune checkpoint blockade treatment by increasing the ratio of effector to regulatory cells within the tumor microenvironment

A Farias,A Soto,F A Goldbaum,C J Goldin,P M Berguer,S Sosa,C Gil,F Puttur

doi:10.1038/s41598-021-94837-7

Abstract

Brucella lumazine synthase (BLS) is a homodecameric protein that activates dendritic cells via toll like receptor 4, inducing the secretion of pro-inflammatory cytokines and chemokines. We have previously shown that BLS has a therapeutic effect in B16 melanoma-bearing mice only when administered at early stages of tumor growth. In this work, we study the mechanisms underlying the therapeutic effect of BLS, by analyzing the tumor microenvironment. Administration of BLS at early stages of tumor growth induces high levels of serum IFN-γ, as well as an increment of hematopoietic immune cells within the tumor. Moreover, BLS-treatment increases the ratio of effector to regulatory cells. However, all treated mice eventually succumb to the tumors. Therefore, we combined BLS administration with anti-PD-1 treatment. Combined treatment increases the outcome of both monotherapies. In conclusion, we show that the absence of the therapeutic effect at late stages of tumor growth correlates with low levels of serum IFN-γ and lower infiltration of immune cells in the tumor, both of which are essential to delay tumor growth. Furthermore, the combined treatment of BLS and PD-1 blockade shows that BLS could be exploited as an essential immunomodulator in combination therapy with an immune checkpoint blockade to treat skin cancer.

Highlights

Brucella lumazine synthase (BLS) is a homodecameric protein that activates dendritic cells via toll like receptor 4, inducing the secretion of pro-inflammatory cytokines and chemokines
Studies employing mouse models of melanoma have shown that targeting certain innate immune signaling pathways, in particular, Toll-Like Receptor (TLR), retinoic acid-inducible gene-I-like receptors (RLR), and stimulator of interferon genes (STING) signaling pathways may prove critical towards aiding in tumor regression either by direct induction of tumor cell apoptosis[32] or by reducing tumor cell proliferation[33]
We have shown that the lack of therapeutic effect when BLS is administered at later stages of tumor growth correlates with the decreased expression of TLR4 in the tumor cell surface

Summary

Introduction

Brucella lumazine synthase (BLS) is a homodecameric protein that activates dendritic cells via toll like receptor 4, inducing the secretion of pro-inflammatory cytokines and chemokines. Immune checkpoint blockade (ICB) has emerged as a promising immunotherapy for cancer patients This therapy enables tumor-reactive T cells to overcome regulatory mechanisms and mount an effective anti-tumoral response by removing inhibitory signals of T-cell activation. TLR agonists induce activation of DC, promote Th1-type immune responses, antigen presentation and type I interferon production[9] This potent immunomodulatory effect of TLR agonists, in combination with tumor-associated antigens can induce a specific anti-tumor response[10,11,12]. Combination strategies using TLR agonists and ICB have been recently proposed The rationale behind this combination is that the increased priming of antigen-presenting cells can potentially overcome resistance to PD-1/PD-L1 blockade and enhance T cell activation. Even though there are several clinical trials ongoing based on TLR agonists and ICB, more research is needed to better understand how ICB can be modulated by TLR agonists

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