BACKGROUND

FDA-approved immune checkpoint inhibitors (ICIs) are the state-of-the-art treatment for mismatch-repair deficient tumors (dMMR). Their immunomodulatory effects in a preventive setting, are poorly studied. In this study, we used two ICIs against PD-L1 or LAG-3 in a preclinical mouse model of dMMR-driven carcinogenesis.

METHODS

Msh2 mice without clinical signs of tumor development (i.e. age <12 weeks) received repeated applications (n = 8, 28-day interval) of anti-PD-L1, anti-LAG-3 (2.5 mg/kg bw, i.p.) or isotype (anti-IgG1, 2.5 mg/kg bw, i.p.). Blood phenotyping, tumor microenvironment, and hematopoiesis, and presence of procoagulant extracellular vesicles (EV) were studied.

RESULTS

Prophylactic ICI application significantly prolonged overall survival (OS) of Msh2 mice (OS: anti-PD-L1: 54.4 wks and anti-LAG-3: 58.4 wks vs. ctrl 35.8 wks). Circulating exhausted and regulatory T cells were significantly lower in the ICI groups. Splenic exhaustion markers showed correlating results. Outgrowing tumors showed an inflammatory response-related gene signature. Accompanying immunofluorescence confirmed data and identified reduced numbers of tumor-infiltrating regulatory granulocytes in late-onset tumors. Alterations in bone marrow hematopoiesis accompanied this massive immune modulation, indicating successful prevention of myeloid-shifted hematopoiesis. Plasma coagulation of EVs was not significantly altered in the ICI groups.

CONCLUSION

Preventive ICI prolongs overall survival of cancer-prone mice. The sustained immune modulation and normal bone marrow hematopoiesis may pre-sensitize late-onset dMMR tumors to a second round of immunotherapy. Hence, we highlight the importance of preventive strategies for germline MMR mutation carriers and recommend improved screening of patients eligible for prophylactic ICIs to improve long-term outcomes.