Chitin-mediated blockade of chitinase-like proteins reduces tumor immunosuppression, inhibits lymphatic metastasis and enhances anti-PD-1 efficacy in complementary TNBC models.

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. Overall design: RNA was isolated from 4T1 and 66cl4 primary tumors at 5 w p.i. using the Rneasy Mini Kit (QiAgen, Valencia, CA, USA). All analyses were done on 5 samples per treatment group from both TNBC models, yielding a total of 40 samples. Only 4T1 tumor sample 3 from the untreated group showed RNA degradation and was therefore excluded from the analyses. RNA sequencing was done in collaboration with Azenta Life Sciences and performed on an Illumina Novaseq system (paired end, read length of 150 bp), aiming for 10 million reads per sample.