Arp2/3 complex-driven spatial patterning of the BCR enhances immune synapse formation, BCR signaling and B cell activation

When B cells encounter antigens on the surface of an antigen-presenting cell (APC), B cell receptors (BCRs) are gathered into microclusters that recruit signaling enzymes. These microclusters then move centripetally and coalesce into the central supramolecular activation cluster of an immune synapse. The mechanisms controlling BCR organization during immune synapse formation, and how this impacts BCR signaling, are not fully understood. We show that this coalescence of BCR microclusters depends on the actin-related protein 2/3 (Arp2/3) complex, which nucleates branched actin networks. Moreover, in murine B cells, this dynamic spatial reorganization of BCR microclusters amplifies proximal BCR signaling reactions and enhances the ability of membrane-associated antigens to induce transcriptional responses and proliferation. Our finding that Arp2/3 complex activity is important for B cell responses to spatially restricted membrane-bound antigens, but not for soluble antigens, highlights a critical role for Arp2/3 complex-dependent actin remodeling in B cell responses to APC-bound antigens.

Human herpesvirus 8 (KSHV) contamination of peripheral blood and autograft products from multiple myeloma patients

Human herpesvirus 8 (HHV-8), also known as Kaposi's sarcoma-associated herpesvirus (KSHV), has recently been identified within the bone marrow dendritic cells of multiple myeloma (MM) patients. This virus contains homologues to human cytokines such as IL-6 that could potentially stimulate myeloma cell growth and contribute to disease pathogenesis. Since mobilization chemotherapy may increase circulating dendritic cell numbers, we searched for HHV-8 in peripheral blood mononuclear cells (PBMCs) before and after mobilization chemotherapy given to MM patients. Furthermore, we determined if autograft purging using the CEPRATE SC device would reduce the percentage of HHV-8 infected stem cell products. Only two of the 39 PBMC samples collected prior to mobilization chemotherapy contained PCR detectable virus, yet nine of 37 PBMCs collected on the first day of leukapheresis had detectable HHV-8 (P = 0.016). HHV-8 was more frequently identified in autograft products before vs after Ceprate SC selection (40% vs 15%, P = 0.016). Although the role HHV-8 plays in myeloma pathogenesis remains unclear, these results imply that mobilization chemotherapy increases the numbers of circulating HHV-8-infected dendritic cells within the peripheral blood. In addition, CD34 selection of autograft products in MM patients may reduce the reintroduction of virally infected cells following high-dose chemotherapy. Bone Marrow Transplantation (2000) 25, 153-160.