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In humans, the stages and dynamics of B cell development after antigen encounter remain unclear. Identifying early B cell differentiation stages could reveal biomarkers for humoral immunity and potential targets to prevent unwanted antibody responses. We characterized antigen©\specific B cell responses longitudinally after SARS©\CoV©\2 mRNA vaccination using multiparameter spectral flow cytometry. Spike©\specific IgG+ CD27+ CD71+ activated B cells (ActBCs), presumed to be germinal center©\derived and IgG+ DN2 extrafollicular B cells, dominated the early antigen©\specific B cell response, while memory B cells were the main population 6 months after vaccination. Within the IgG+ ActBC compartment, we delineated six novel clusters with specific contraction dynamics. Following the second vaccination, certain ActBC clusters displayed sustained expansion over time, being phenotypically similar to memory B cells, while others strongly expanded and subsequently contracted. Several of the rapidly contracting ActBC clusters expressed CD11c, a defining marker for atypical B cells, suggesting a possible extrafollicular origin of these clusters. The transient presence of heterogeneous ActBC clusters was also observed for total B cells when gated in an antigen©\independent manner. Characterization of novel ActBC clusters early after antigen encounter helps delineate and dissect the complexity of B cell differentiation, which is vital for understanding unwanted B cell responses. Characterization of the early antigen©\specific B cell response post©\SARS©\CoV©\2 vaccination reveals novel activated B cell clusters, showing different phenotypes and contraction dynamics. Some short©\lived activated B cells expressed both CD71 and the extrafollicular marker CD11c. These results advance our understanding of B©\cell differentiation regulation and biomarker potential.

AbstractBackgroundPeritoneal metastasis (PM) is one of the most challenging clinical problems in gastric cancer (GC), largely due to its high recurrence rate and poor response to current therapies. Increasing evidence indicates that remodeling of the extracellular matrix (ECM) plays an important role in therapeutic failure. However, how specific stromal¨Cimmune interactions contribute to PM heterogeneity and immunotherapy resistance remains unclear. In this study, we investigated how ECM composition¡ªparticularly the accumulation of hyaluronic acid (HA)¡ªinfluences the immune microenvironment and therapeutic responses in GC-associated PM.MethodsWe combined histopathological assessment, analyses of patient-derived specimens, single-cell transcriptomic profiling, and murine models of PM to delineate ECM remodeling patterns and immune cell dynamics in therapy-sensitive and therapy-resistant lesions. In addition, functional assays and pharmacological approaches were used to examine HA¨CCD44 signaling and its impact on CD4+ T cell differentiation and responsiveness to immune checkpoint blockade.ResultsTherapy-sensitive PM lesions were characterized by enrichment of elastic fibers, whereas therapy-resistant lesions showed collagen accumulation. Notably, HA deposition emerged as a key feature distinguishing these ECM states and was closely associated with differential therapeutic outcomes. Elevated HA levels activated CD44-dependent signaling in CD4+ T cells, driving regulatory T cell (Treg) differentiation through a CD44¨CIQGAP1¨CRAC1¨CSMAD3 signaling pathway and thereby establishing an immunosuppressive microenvironment. Importantly, pharmacological inhibition of CD44 reduced Treg expansion and markedly enhanced the antitumor efficacy of anti-PD-1 therapy in murine PM models.ConclusionsOur findings identify HA¨CCD44 signaling as a critical link between ECM remodeling and immune evasion in GC PM. Targeting ECM-driven immunosuppressive mechanisms may represent a promising strategy to overcome therapeutic resistance and improve the efficacy of immunotherapy in this aggressive disease.

Chronic hemophilic synovitis (CHS), driven by hemosiderin-laden macrophages from recurrent hemarthrosis, is a major cause of joint damage in hemophilia. Platelet-rich plasma (PRP) is a promising regenerative therapy for joint diseases. This study investigated PRP¡¯s ability to modulate macrophage polarization from a pro-inflammatory (M1) to a pro-resolving, tissue-repairing (M2) phenotype in CHS. We analyzed synovial fluid (SF) from CHS patients (N = 22), both pre- and post-PRP treatment. Ex vivo analysis revealed a predominant M1 profile with an increased proportion of CD11+CD14+CD64hi compared with CD206+ or CD163+ M2 macrophages in CHS SF. In vitro experiments showed that CHS SF skewed monocyte-derived macrophages toward an M1 inflammatory program, evaluated by flow cytometry, qPCR, and ELISA. However, adding PRP significantly modulated the pro-inflammatory macrophage program, promoting an M2 tissue repair profile. Furthermore, a random forest machine learning algorithm, applied to public scRNAseq data, confirmed PRP¡¯s macrophage reprogramming effect. Functional assays also showed increased TGF-¦Â secretion and macrophage fusion when challenged with neutrophil extracellular traps (NETs). A small patient follow-up cohort treated with intra-articular PRP showed similar results, including normalization of cellular content and reduced CD64/CD206 expression. These findings indicate that PRP treatment effectively shifts SF-associated M1 macrophages to an M2-like phenotype, highlighting its potential as a therapeutic strategy for CHS.