Alkylating Agent-Induced ER Stress Overcomes Microenvironmental Resistance to Lymphoma Therapy

Compartment-specific resistance to cancer therapy remains poorly understood. We utilized orthotopic xenografts of human double-hit lymphoma (DHL) to interrogate responses across involved sites. We identified resistance to multiple chemotherapies and the anti-CD52 antibody Alemtuzumab within the bone marrow (BM) that depended on extensive lymphoma involvement and impaired antibody-dependent cellular phagocytosis. This resistance was overcome by high doses of alkylating agents, including cyclophosphamide (CTX), which exhibited >80-fold in vivo synergy with Alemtuzumab. CTX induced ER stress in BM DHL cells leading to ATF4- mediated paracrine secretion of VEGF-A and massive macrophage infiltration. Macrophages from DHL-engrafted, CTX-treated mice had increased phagocytic capacity for lymphoma cells that was reversed by VEGF-A blockade and required SYK phosphorylation. A subset of these macrophages, defined by surface CD36/FcgRIV and a distinct transcriptional state, were “superphagocytic.” Together, our findings define a unique mechanism through which high-dose alkylating agents can overcome therapy-resistant niches by ER stress-induced activation of phagocytosis.