MSC CM can block IgG secretion from PCs. (A) Live MSCs can block IgG secretion from splenocytes. Splenocytes collected from OVA-immunized mice were either cultured in vitro in splenocyte media or with MSC 24 hours before ELISPOT read-outs. For the L-NAME test group, MSCs were treated for 24 hours with the inhibitor before the addition of splenocytes; 4% paraformaldehyde (PFA) was used to fix MSC. Only metabolically active MSC, even in the presence of L-NAME, had IgG-inhibitory properties. Fixing MSC led to loss in inhibition (n = 3/group; P < .001). (B) MSCs secrete a soluble inhibitory factor(s). Based on the observation made previously, MSC CM was used alone in ELISPOT to demonstrate that it contains soluble factor(s) mediating IgG inhibition (n = 3/group; P < .001). (C) WT MSC led to MMP-cleaved CCL2. CM derived from WT MSC or CCL2^−/− MSC was run on a Tricine SDS-PAGE to detect CCL2. (D) MSCs do not express CCR2. RNA extracted from MSCs was used in an RT-PCR to demonstrate the absence of CCR2 expression on MSCs, suggesting no autocrine loop induced by CCL2/CCR2 interaction.