Hematopoietic stem cells (HSCs) undergo a functional switch in neonatal mice hallmarked by a decrease in self-renewing divisions and entry into quiescence. Here, we investigated whether the developmental attenuation of B-1a cell output is a consequence of a shift in stem cell state during ontogeny. Using cellular barcoding for in vivo single-cell fate analyses, we found that fetal liver definitive HSCs gave rise to both B-1a and B-2 cells. Whereas B-1a potential diminished in all HSCs with time, B-2 output was maintained. B-1a and B-2 plasticity could be reinitiated in a subset of adult HSCs by ectopic expression of the RNA binding protein LIN28B, a key regulator of fetal hematopoiesis, and this coincided with the clonal reversal to fetal-like elevated self-renewal and repopulation potential. These results anchor the attenuation of B-1a cell output to fetal HSC behavior and demonstrate that the developmental decline in regenerative potential represents a reversible HSC state.

The ability of B-1 cells to become positively selected into the mature B cell pool, despite being weakly self-reactive, has puzzled the field since its initial discovery. Here, we explore changes in B cell positive selection as a function of developmental time by exploiting a link between CD5 surface levels and the natural occurrence of self-reactive B cell receptors (BCRs) in BCR wild-type mice. We show that the heterochronic RNA binding protein Lin28b potentiates a neonatal mode of B cell selection characterized by enhanced overall positive selection in general and the developmental progression of CD5+ immature B cells in particular. Lin28b achieves this by amplifying the CD19/PI3K/c-Myc positive feedback loop, and ectopic Lin28b expression restores both positive selection and mature B cell numbers in CD19−/− adult mice. Thus, the temporally restricted expression of Lin28b relaxes the rules for B cell selection during ontogeny by modulating tonic signaling. We propose that this neonatal mode of B cell selection represents a cell-intrinsic cue to accelerate the de novo establishment of the adaptive immune system and incorporate a layer of natural antibody-mediated immunity throughout life.

SignificanceTo provide optimal host defense, the full spectrum of antibody-based immunity requires natural antibodies and immunization-induced antigen-specific antibodies. Here we show that the PTIP (Pax transactivation domain-interacting protein) chromatin regulator is induced by B cell activation to potentiate the establishment of steady-state and postimmune serum antibody levels. It does so by promoting activation-associated proliferation and differentiation of all the major B cell subsets, at least in part, through regulating the NF-κB pathway. With the genetic basis still unknown for a majority of patients with common variable immunodeficiency, further work investigating how PTIP controls cell signaling may generate valuable new insight for human health and disease.

No abstract

IL-1, IL-33 and IL-36 have complementary pro-inflammatory roles during the immune response promoting autoimmune and inflammatory diseases, such as myocarditis. Therapeutic blockers targeting the IL-1 pathway have previously been developed. However, we hypothesized that blockade of the shared co-receptor of these three pathways, IL-1 receptor accessory protein (IL1RAP), would exhibit a more potent and broader anti-inflammatory profile. To investigate this hypothesis, we induced coxsackievirus B3 (CVB3)-mediated or experimental autoimmune myocarditis (EAM) in BALB/c mice, followed by treatment with mCAN10, an Fc-modified IL1RAP-blocking monoclonal antibody. IL1RAP blockade strongly reduced the severity of acute viral myocarditis when compared to isotype, saline or IL-1 receptor antagonist (IL1Ra) treatment (the gold standard of IL-1 receptor blockade), without affecting viral clearance from the heart. Spectral flow cytometry of heart immune populations showed that mCAN10 significantly reduced the infiltrating numbers of several populations, especially effector CD4+ and CD8+ T cells, inflammatory Ly6C+CCR2+ monocytes, neutrophils and eosinophils. Spatial gene expression revealed reduction in canonical inflammatory gene and pathway expression in the cardiac immune foci. This translated into protection from cardiac function deterioration and ejection fraction loss during viral and experimental autoimmune myocarditis. Altogether, our data show that a monoclonal antibody targeting 3 pro-inflammatory cytokine systems simultaneously, IL-1, IL-33 and IL-36, potently reduces viral and autoimmune myocarditis severity in a broad fashion not recapitulated by IL-1 blockade, and shows translational promise for development into therapeutics against cardiac inflammatory disease.