ABSTRACT: Purpose: Evaluate the anti-tumorigenic efficacy and immunomodulatory effects of chitin as general blocker of immunosuppressive chitinase-like proteins (CLPs) in combination with and without anti-PD-1 immune checkpoint blockade (ICB) in an immunocompetent 4T1- and 66cl4-based intraductal model for triple-negative breast cancer (TNBC). Methods: 4T1 and 66cl4 mammary tumor-bearing female BALB/c mice were either left untreated or were treated with anti-PD-1, chitin or chitin + anti-PD-1 for 2 weeks (w) between 3 and 5 w post-inoculation (p.i.) of the mammary tumor cells. After the 2-w treatment, 4T1 and 66cl4 primary tumors were resected and RNA was isolated from the tissue using in-house developed protocols. Results: Chitin-mediated reduction in innate immunosuppression and increased anti-tumor T-cell immunity in primary tumors of both TNBC models was clearly presented at the genomic level based on RNA-sequencing analysis. More specifically, a selection of 68 and 55 innate immunity-related genes in respectively 4T1 and 66cl4 primary tumors were downregulated upon chitin treatment in combination with or without anti-PD-1. Several of the selected innate immunity genes could be linked to immunosuppressive myeloid cell types, including Ccl2, Cxcl2, Csf1r and Itgam. Genes related to T-cell exhaustion, including Pdcd1, Cd274, Havcr2 and Lag3, were downregulated upon chitin and chitin + anti-PD-1 treatment in 4T1 tumors. Chitin- and chitin + anti-PD-1-treated 66cl4 tumors showed upregulation of genes associated with enhanced T-cell activity, including Cd8a, Cd8b1, Ifng, Il12b and Il18r1. In line with an overall reduced inflammation upon chitin treatment at the protein level, predominantly through myeloid cell reduction, a selected set of 68 and 48 inflammation-related genes were downregulated in chitin- and chitin + anti-PD-1-treated primary tumors of respectively the 4T1- and the 66cl4-based model. Importantly, genes that have been associated with the signaling and pro-tumorigenic activity of a prominent CLP family member, i.e. CHI3L1, were also downregulated in primary tumors following chitin treatment either with or without anti-PD-1, including Usf1 and Rab37 in the 4T1-based model and Lgals3 in the 66cl4-based model. Conclusion: Our findings highlight that chitin, as a general CLP blocker, reduces cancer-associated immunosuppression and enhances anti-tumor immunity as well as ICB responses in complementary ICB-resistant TNBC models, supporting its potential clinical relevance in immunosuppressed TNBC patients.
