Depending on the stimuli they encounter, B lymphocytes engage in signaling events that lead to immunity or tolerance. Both responses are mediated through antigen interactions with the B cell antigen receptor (BCR). Antigen valency is thought to be an important parameter in B cell signaling, but systematic studies are lacking. To explore this issue, we synthesized multivalent ligands of defined valencies using the ring-opening metathesis polymerization (ROMP). When mice are injected with multivalent antigens generated by ROMP, only those of high valencies elicit antibody production. These results indicate that ligands synthesized by ROMP can activate immune responses in vivo. All of the multivalent antigens tested activate signaling through the BCR. The ability of antigens to cluster the BCR, promote its localization to membrane microdomains, and augment intracellular Ca2+ concentration increases as a function of antigen valency. In contrast, no differences in BCR internalization were detected. Our results indicate that differences in the antigenicity of BCR ligands are related to their ability to elicit increases in intracellular Ca2+ concentration. Finally, we observed that unligated BCRs cluster with BCRs engaged by multivalent ligands, a result that suggests that signals mediated by the BCR are amplified through receptor arrays. Our data suggest a link between the mechanisms underlying signal initiation by receptors that must respond with high sensitivity.

Tendon healing follows a complex series of coordinated events, which ultimately produces a mechanically inferior tissue more scar‐like than native tendon. More regenerative healing occurs when anti‐inflammatory M2 macrophages play a more dominant role. Mesenchymal stromal/stem cells (MSCs) are able to polarize macrophages to an M2 immunophenotype via paracrine mechanisms. We previously reported that coculture of CD14+ macrophages (MQs) with MSCs resulted in a unique M2‐like macrophage. More recently, we generated M2‐like macrophages using only extracellular vesicles (EVs) isolated from MSCs creating “EV‐educated macrophages” (also called exosome‐educated macrophages [EEMs]), thereby foregoing direct use of MSCs. For the current study, we hypothesized that cell therapy with EEMs would improve in vivo tendon healing by modulating tissue inflammation and endogenous macrophage immunophenotypes. We evaluated effects of EEMs using a mouse Achilles tendon rupture model and compared results to normal tendon healing (without any biologic intervention), MSCs, MQs, or EVs. We found that exogenous administration of EEMs directly into the wound promoted a healing response that was significantly more functional and more regenerative. Injured tendons treated with exogenous EEMs exhibited (a) improved mechanical properties, (b) reduced inflammation, and (c) earlier angiogenesis. Treatment with MSC‐derived EVs alone were less effective functionally but stimulated a biological response as evidenced by an increased number of endothelial cells and decreased M1/M2 ratio. Because of their regenerative and immunomodulatory effects, EEM treament could provide a novel strategy to promote wound healing in this and various other musculoskeletal injuries or pathologies where inflammation and inadequate healing is problematic. Stem Cells 2019;37:652–662

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Nuclear factor-κB (NF-κB) is a family of transcription factors that play a key role in cell survival and proliferation in many hematological malignancies, including multiple myeloma (MM). Bortezomib, a proteasome inhibitor used in the management of MM, can inhibit both canonical and noncanonical activation of NF-κB in MM cells. However, we previously reported that a significant fraction of freshly isolated MM cells harbor bortezomib-resistant NF-κB activity. Here, we report that hyaluronan and proteoglycan link protein 1 (HAPLN1) is produced in bone marrow stromal cells from MM patients, is detected in patients' bone marrow plasma, and can activate an atypical bortezomib-resistant NF-κB pathway in MM cells. We found that this pathway involves bortezomib-resistant degradation of the inhibitor of NF-κB (IκBα), despite efficient bortezomib-mediated inhibition of proteasome activity. Moreover, HAPLN1 can also confer bortezomib-resistant survival of MM cells. We propose that HAPLN1 is a novel pathogenic factor in MM that induces an atypical NF-κB activation and thereby promotes bortezomib resistance in MM cells.