Emerging roles for multipotent, bone marrow–derived stromal cells in host defense

Multipotent, bone marrow–derived stromal cells (BMSCs, also known as mesenchymal stem cells [MSCs]), are culture-expanded, nonhematopoietic cells with immunomodulatory effects currently being investigated as novel cellular therapy to prevent and to treat clinical disease associated with aberrant immune response. Emerging preclinical studies suggest that BMSCs may protect against infectious challenge either by direct effects on the pathogen or through indirect effects on the host. BMSCs may reduce pathogen burden by inhibiting growth through soluble factors or by enhancing immune cell antimicrobial function. In the host, BMSCs may attenuate pro-inflammatory cytokine and chemokine induction, reduce pro-inflammatory cell migration into sites of injury and infection, and induce immunoregulatory soluble and cellular factors to preserve organ function. These preclinical studies provide provocative hints into the direction MSC therapeutics may take in the future. Notably, BMSCs appear to function as a critical fulcrum, providing balance by promoting pathogen clearance during the initial inflammatory response while suppressing inflammation to preserve host integrity and facilitate tissue repair. Such exquisite balance in BMSC function appears intrinsically linked to Toll-like receptor signaling and immune crosstalk.