Inducible [CCR2.sup.+] nonclassical monocytes mediate the regression of cancer metastasis

A major limitation of immunotherapy is the development of resistance resulting from cancer-mediated inhibition of host lymphocytes. Cancer cells release CCL2 to recruit classical monocytes expressing its receptor CCR2 for the promotion of metastasis and resistance to immunosurveillance. In the circulation, some CCR2-expressing classical monocytes lose CCR2 and differentiate into intravascular nonclassical monocytes that have anticancer properties but are unable to access extravascular tumor sites. We found that in mice and humans, an ontogenetically distinct subset of naturally underrepresented CCR2-expressing nonclassical monocytes was expanded during inflammatory states such as organ transplant and COVID-19 infection. These cells could be induced during health by treatment of classical monocytes with small-molecule activators of NOD2. The presence of CCR2 enabled these inducible nonclassical monocytes to infiltrate both intra- and extravascular metastatic sites of melanoma, lung, breast, and colon cancer in murine models, and they reversed the increased susceptibility of [Nod2.sup.-/-] mutant mice to cancer metastasis. Within the tumor colonies, [CCR2.sup.+] nonclassical monocytes secreted CCL6 to recruit NK cells that mediated tumor regression, independent of T and B lymphocytes. Hence, pharmacological induction of [CCR2.sup.+] nonclassical monocytes might be useful for immunotherapy-resistant cancers.