Recombinant CALR Polarizes and Activates Macrophages in Tumors

ABSTRACT

We have created a recombinant form of calreticulin to serve as an immune adjuvant. For many patients with immunogenically cold tumors, immunotherapies are ineffective. Simulating immunogenic cell death with damage-associated molecular patterns (DAMPs) has the potential to increase infiltration and activation of immune cells, characteristics of immunogenically hotter tumors. We hypothesized that delivery of recombinant calreticulin (CALR), a common DAMP, triggers leukocyte infiltration and activates tumor-infiltrating macrophages. To test this hypothesis, we developed a method to produce CALR with engineered Salmonella. In culture, recombinant CALR rapidly shifted macrophages to an inflammatory M1 phenotype. Media from CALR-stimulated macrophages activated dendritic cells and induced expression of B7 costimulatory markers, which are required for T cell stimulation. In mice, recombinant CALR reduced tumor volume and increased the number of leukocytes and inflammatory macrophages that express B7 costimulatory markers. Recombinant CALR also increased T cell infiltration and the number of activated helper T cells. The observed reduction in tumor growth may have been caused by macrophage polarization and activation of helper T cells. Overall, the administration of recombinant CALR induces artificial ICD, which has the potential to increase antigen presentation, promote infiltration of cytotoxic T cells, and amplify the efficacy of immune therapy in general.