TLR agonists selectively promote terminal plasma cell differentiation of B cell subsets specialized in thymus-independent responses.

Naive murine B cells are known to proliferate and differentiate in response to LPS or CpG, which bind to TLR4 and TLR9, respectively. However, the naive murine B cell compartment is heterogeneous and comprises four different B cell subsets: B-1a, B-1b, marginal zone (MZ), and follicular (FO) B cells. B-1a, B-1b, and MZ B cells are specialized in the response to thymus-independent Ag, and FO B cells are involved in the response to thymus-dependent Ag. This study was undertaken to compare those four naive B cell subsets for their responses to TLR agonists. Quantitative RT-PCR analysis revealed that expression of TLR transcripts differs quantitatively but not qualitatively from one subset to the other. All TLR agonists, with the exception of flagellin and poly(I:C), stimulate B cell proliferation whatever the subset considered. However, TLR ligation leads to massive differentiation of B-1 and MZ B cells into mature plasma cells (PC) but only marginally promotes PC differentiation of FO B cells. Moreover, TLR stimulation strongly up-regulates expression of Blimp-1 and XBP-1(S), two transcription factors known to be instrumental in PC differentiation, in B-1 and MZ B cells but not in FO B cells. Altogether, our findings suggest that B-1 and MZ B cells are poised to PC differentiation in response to the microbial environment and that TLR agonists can be instrumental in stimulating Ab-mediated innate immune protection during microbial infections.

Murine dendritic cell transdifferentiation into osteoclasts is differentially regulated by innate and adaptive cytokines.

Dendritic cells (DC) are the mononuclear cells that initiate adaptive immune responses. Osteoclasts (OC) are the multinucleated giant cells that resorb bone. As previously described for human conventional DC (cDC), we demonstrate that murine cDC, either in vitro generated from Fms-like tyrosine kinase 3 (Flt3)+ bone marrow progenitors or ex vivo purified from spleen, are able to develop into OC in response to M-CSF and receptor activator of NF-kappaB ligand (RANKL) in vitro. This transdifferentiation is driven by the immune environment that controls cDC maturation, cell fusion, tartrate-resistant acid phosphatase (TRAP) and bone resorption activities. Only immature cDC have the capacity to become OC since mature cDC or plasmacytoid DC do not. Additions of the pro-inflammatory cytokines, such as IL-1beta and TNF-alpha, or human rheumatoid synovial fluid, increase murine cDC transdifferentiation into OC, whereas IFN-alpha inhibits it. The adaptive cytokine, IFN-gamma, inhibits cDC fusion while IL-4 increases it. IL-2, IFN-gamma and IL-4 inhibit TRAP and bone resorption activities contrary to IL-10, which enhances both activities. A putative new "immune multinucleated giant cell" unable to resorb bone, which is formed owing to IL-4, is underlined. The future analysis of cDC transdifferentiation into OC in murine models of inflammatory arthritis will give us the quantitative importance of this phenomenon in vivo.

The endoplasmic reticulum is a key component of the plasma cell death pathway.

Plasma cells (PC) are the effector cells of the humoral Ab response. Unlike other dedicated secretory cells, they exist as two populations with opposite cell fates: short-lived and long-lived PC. Upon transformation they lead to an incurable neoplasia called multiple myeloma. In this study we have explored the molecular mechanism of PC death. Our data show that their apoptotic pathway is unique among other hemopoietic cells inasmuch as neither the death receptors nor the mitochondria play the central role. PC apoptosis is initiated by activation of Bax at the endoplasmic reticulum membrane and subsequent activation of the endoplasmic reticulum-associated caspase-4 before the release of mitochondrial apoptogenic factors. Together, our observations indicate that the cardinal function of PC (i.e., Ig secretion) is also the cause of their death.

Cutting edge: immediate RANTES secretion by resting memory CD8 T cells following antigenic stimulation.

The efficiency of CD8 memory response relies partially on the modification of cellular functional capacities. To identify effector functions that can be modified following priming, we have compared the chemokines produced by naive and memory CD8 T cells. Our results show that in contrast to naive cells, resting memory CD8 T cells contain high levels of RANTES mRNA. As a result, they have the capacity to rapidly secrete RANTES upon ex vivo antigenic stimulation. In contrast to that of IFN-gamma, RANTES secretion is mainly due to the translation of the pre-existing mRNA.

BCR ligation reprograms B cells for migration to the T zone and B-cell follicle sequentially.

We have studied the impact of B-cell receptor (BCR) or CD40 ligation on the in vitro chemotactic response of tonsillar B cells to 4 chemokines: stromal cell-derived factor (SDF)-1alpha, macrophage inflammatory protein (MIP)-3alpha, MIP-3beta, and B-cell-attracting chemokine (BCA)-1. In the tonsil, SDF-1 and MIP-3alpha are both expressed in the crypt epithelium, while MIP-3beta is found in the T zone and BCA-1 in the follicles. Resting virgin and memory B cells display a similar chemotaxis pattern, and they both have the potential to migrate in vitro to all 4 chemokines studied. This pattern of responsiveness is strongly modified by a surrogate antigen (Ag) but is not altered by CD40 ligand. We report here that surrogate Ag induces a profound and sustained suppression of the response to the crypt chemokines SDF-1alpha and MIP-3alpha, while it exacerbates the migratory response to MIP-3beta. The effect of surrogate Ag on the response to BCA-1 is biphasic: After an initial phase of suppression, chemotaxis toward BCA-1 is strongly up-regulated. Our results suggest that Ag is primarily responsible for reprogramming the B-cell chemotaxis responsiveness during the humoral response. We propose that it initiates an ordered change of the chemotaxis machinery allowing Ag-activated B cells to relocate in the T zone and B-cell follicles sequentially.