Marrow-Derived Mesenchymal Stromal Cells Can Block an Antigen-Driven Humoral Response by Suppressing Plasma Cell Activity Via Production of CCL2.

Marrow-derived Mesenchymal Stromal Cells (MSCs) have been demonstrated to possess powerful immunomodulatory suppressive properties. In vitro studies by many groups have shown that MSCs can suppress th1 immune responses as exemplified by in vitro blockade of a 2-way mixed lymphocyte reaction (MLR) by an assortment of mechanisms including production of soluble factors such as: nitric oxide, transforming growth factor-β, IGF-1 and hepatocyte growth factor. Lately, interesting studies have also demonstrated the potency of MSCs in modulating humoral immunity by inhibiting B-cell migration, proliferation as well as immunoglobulin secretion in vitro. We here sought to further define the ability of autologous MSCs in modulating an ovalbumin (OVA) antigen-specific humoral response in normal immune competent C57BL/6 mice. Immunologically naïve mice were vaccinated with 50 ug of recombinant chicken ovalbumin protein followed by a boost dose 2 weeks later. All mice developed a robust IgM and IgG anti-OVA antibody response as measured in serial plasma samples over time. Following establishment of anti-OVA humoral immunity, test mice (n=10) were administered 1.5 million autologous MSCs in the peritoneal cavity twice at one month interval. Compared with same-treated controls (n=10), we found that MSC treated mice significantly suppressed anti-OVA IgG titer. This inhibitory effect requires metabolically active MSCs since live MSCs inhibited anti-OVA IgG secretion in ELISPOT assays, whereas paraformaldehyde fixed MSCs had no effect. Interestingly, the addition of MSC conditioned media directly on spleen-derived plasma cells derived form OVA immunized mice inhibited OVA-specific IgG secretion in vitro. A cytokine array screen on MSC secretome identified CCL2 as a possible effector molecule in suppressing plasma cell activity, and we found that anti-CCL2 neutralizing antibodies abolished the suppressive effect of MSCs on plasma cells. In conclusion, we have found that MSCs can suppress a pre-established humoral response to a defined antigen in vivo. This effect is contact independent, and requires metabolically active MSCs. MSC-derived CCL2 appears to be a key suppressor of antigen-specific immunoglobulin in this system.