Early Emergence of CD19-Negative Human Antibody-Secreting Cells at the Plasmablast to Plasma Cell Transition.

Long-lived human plasma cells (PCs) play central roles in immunity and autoimmunity and are enriched among the subpopulation of CD19neg human PCs. However, whether human CD19neg PCs are necessarily aged cells that have gradually lost CD19 expression is not known. Assessing peripheral blood samples at steady-state and during the acute response to influenza vaccination in healthy donors, we identify the presence of phenotypic CD19neg plasmablasts, the proliferative precursor state to mature PCs, and demonstrate by ELISPOT that these are Ab-secreting cells (ASCs). During the acute response to influenza vaccination, CD19pos, CD19low, and CD19neg ASCs secrete vaccine-specific Abs and show linked IGHV repertoires. To address precursor/product relationships, we use in vitro models that mimic T-dependent and T-independent differentiation, finding that the CD19neg state can be established at the plasmablast to PC transition, that CD19neg PCs increase as a percentage of surviving PCs in vitro, and that CD19neg and CD19pos PCs can be maintained independently. These data provide proof-of-principle for the view that newly generated ASCs can acquire a mature PC phenotype that is accompanied by loss of CD19 expression at an early stage of differentiation and that aging is not an obligate requirement for a CD19neg state to be established.

Network Analysis Identifies Proinflammatory Plasma Cell Polarization for Secretion of ISG15 in Human Autoimmunity.

Plasma cells (PCs) as effectors of humoral immunity produce Igs to match pathogenic insult. Emerging data suggest more diverse roles exist for PCs as regulators of immune and inflammatory responses via secretion of factors other than Igs. The extent to which such responses are preprogrammed in B-lineage cells or can be induced in PCs by the microenvironment is unknown. In this study, we dissect the impact of IFNs on the regulatory networks of human PCs. We show that core PC programs are unaffected, whereas PCs respond to IFNs with distinctive transcriptional responses. The IFN-stimulated gene 15 (ISG15) system emerges as a major transcriptional output induced in a sustained fashion by IFN-α in PCs and linked both to intracellular conjugation and ISG15 secretion. This leads to the identification of ISG15-secreting plasmablasts/PCs in patients with active systemic lupus erythematosus. Thus, ISG15-secreting PCs represent a distinct proinflammatory PC subset providing an Ig-independent mechanism of PC action in human autoimmunity.

Amino acid deprivation links BLIMP-1 to the immunomodulatory enzyme indoleamine 2,3-dioxygenase.

Catabolism of tryptophan by IDO1 plays an important role in the control of immune responses. Activation of the eukaryotic initiation factor 2alpha (eIF2alpha) kinase general control nonderepressible-2 (GCN2) following tryptophan depletion is a major pathway mediating this effect. However, immunomodulatory target genes of GCN2 activation are poorly defined. The transcriptional repressor B lymphocyte-induced maturation protein-1 (BLIMP-1) is a target of the eIF2alpha kinase1, protein kinase-like ER kinase (PERK) during the unfolded protein response of the endoplasmic reticulum. Thus, BLIMP-1 might also be a mediator of the GCN2 stress response pathway activated by IDO1 and tryptophan depletion. Indeed, in human monocytes BLIMP-1 mRNA and protein are up-regulated in response to both a pharmacological activator of GCN2 and tryptophan-depletion generated by IDO1-transfected cells. This suggests a functional role for BLIMP-1 in the immunomodulatory effects of the IDO1-GCN2 axis. BLIMP-1 has been shown to repress IFN-gamma-regulated promoters. As IDO1 is itself highly responsive to IFN-gamma, we hypothesized that BLIMP-1 functions in a feedback loop to regulate IDO1 expression. We found that BLIMP-1 binds to IFN-responsive sites in the IDO1 promoter and represses IFN-dependent IDO1 activation. We propose that BLIMP-1 acts in a negative feedback loop to successfully balance the outcome of tolerance vs inflammation.

Interaction surface of the Spo0A response regulator with the Spo0E phosphatase.

Spo0A~P is the essential response regulator and transcription factor for sporulation initiation in Bacillus subtilis. The phosphorylation level of Spo0A in the cell is determined by the sensor kinase activity of the phosphorelay, donating phosphoryl groups, and the antagonistic effects of dephosphorylation mediated by the Rap and Spo0E families of phosphatases. In this study, spo0A mutations were generated that encoded proteins less sensitive to the activity of Spo0E than the wild-type protein. The Spo0A substitutions N12K, P60S, L62P and F88L are surface exposed and localize to the same face of the molecule as the active site and in its close proximity on the beta1-alpha1, beta3-alpha3 and beta4-alpha4 loops. The corresponding surface in the Spo0F response regulator was shown previously to be involved in the interaction with the RapB phosphatase, as well as the KinA histidine kinase and the Spo0B phosphotransferase. Thus, residues occupying the same position (N12:Q12, F88:Y84) and the same loops in Spo0A or Spo0F are involved in the interaction with the structurally unrelated Spo0E and RapB phosphatases, respectively, in addition to kinases and phosphotransferase. The specificity in phosphatase target recognition must be the result of side-chain variability within the response regulators and the interactions they promote. The residues involved in Spo0E interaction are identical in all Spo0A orthologues from spore-forming Bacilli encoding Spo0E phosphatases.