Highlights d Antibody blockade of MerTK prevents apoptotic cell clearance by macrophages d MerTK blockade induces tumor-cGAS-and host-STINGdependent type I IFN response d Extracellular ATP facilitates transfer of tumor-derived cGAMP to TAMs via P2X7R d MerTK blockade increases tumor immunogenicity and enhances anti-PD-1/PD-L1 therapy

The thymus contains a population of B cells that colocalize with dendritic cells and medullary thymic epithelial cells in the thymic medulla. The development and functional significance of these cells are largely unknown. Using recombination-activating gene 2 GFP reporter mice along with parabiosis experiments, we demonstrate that the vast majority of thymic B cells develop from progenitors within the thymus. Thymic B cells express unique phenotypic markers compared with peripheral B cells; particularly they express high levels of MHC class II, suggesting that they are poised to present selfantigens efficiently. Using Ig knock-in and T-cell receptor transgenic mice specific for the self-antigen glucose-6-phosphate isomerase, we show that autoreactive thymic B cells serve as efficient antigen-presenting cells for T cell negative selection even when they are present at low frequencies. Furthermore, the endogenous thymic B-cell repertoire also functions in this capacity. These results suggest that developing thymic B cells could efficiently capture a broad array of autoantigens through their B-cell receptors, presenting peptides derived from those autoantigens to developing thymocytes and eliminating cognate T cells.N egative selection purges autoreactive T cells from the immune repertoire and is the major mechanism of central tolerance in the thymus. This process depends on presentation of self-peptides to developing thymocytes by antigen-presenting cells (APCs). The question of which APC presents self-antigen for negative selection has been investigated extensively. Initial studies using bone marrow chimeras found that bone-marrowderived hematopoietic cells are required for negative selection (reviewed in refs. 1 and 2). Many subsequent studies have demonstrated that cortical and medullary thymic epithelial cells (mTECs) can be quite efficient for negative selection as well (1-3). The role of mTECs in deleting T cells specific for tissue-restricted antigens has been highlighted by autoimmunity in both humans and mice possessing mutations in the AIRE gene, which controls the expression of tissue-specific self-antigens in mTECs (4).Bone-marrow-derived APCs include dendritic cells (DCs), B cells, and macrophages. In vitro assays comparing their relative antigen presentation efficiency showed that DCs were the most efficient, leading to the conclusion that DCs were largely responsible for negative selection in the thymus (5). Although B cells are poor at presenting antigens via nonspecific uptake, they capture and internalize cognate antigens that are bound by their B-cell receptors and present them very efficiently (6, 7). Therefore, antigen-specific B cells could be the most efficient APC on a per cell basis for a particular antigen.The thymus contains a small population of B cells that make up around 0.1-0.5% of thymocytes (8-12), similar to the proportion of DCs and mTECs in the thymus (13-15). The origin of thymic B cells has been debated, and development from intrathymic progenitors and migration from the perip...

Arthritis in the K/BxN mouse model is provoked by pathogenic antibodies (Abs) directed against a ubiquitously expressed protein, glucose-6-phosphate isomerase (GPI). To begin dissecting the repertoire of arthritogenic immunoglobulins (Igs) in the K/BxN model, and to provide a basis for comparison with RA patientswe have generated anti-GPI monoclonal Abs (mAbs) from spontaneously activated B cells in the lymphoid organs of arthritic mice. B cell clones with anti-GPI specificities were present at extraordinarily high frequencies in the spleen, and less frequently in other lymphoid organs and in the synovial fluid. None of the anti-GPI mAbs induced arthritis when injected individually into healthy recipients, but most were effective when combined in pairs or larger pools. Arthritogenic combinations depended on mAbs of the IgG1 isotype, which bound to GPI with Kd in the 10−9 M range, with no indication of cooperative binding between complementing pairs. Pathogenicity was not associated with recognition of a particular epitope, but the ability to form mAb/GPI multimers by simultaneous recognition of different epitopes was clearly required, consistent with the known role of complement and FcRs in this model. Sequence analysis revealed structural similarities amongst the mAbs, indicating that a particular subset of B cells may evade tolerance in K/BxN mice, and that affinity maturation by somatic mutation likely takes place. These results confirm that GPI itself, rather than a cross-reactive molecule, is the target of pathogenic Igs.