B Cell Master Regulator See article p. 481

MHC March-Ing Along See article p. 494

An Unlikely Fibrosis Fellow See article p. 524

SARS-CoV-2–Specific T Cells See article p. 580

The germinal center (GC) is the site where B cells undergo differentiation and fate determination; however, the precise mechanisms driving this process are not well understood. Lee et al. (p. 481) show that glycogen synthase kinase 3 (GSK3) is critical for GC B cell differentiation into Ab-secreting plasma cells (PCs). Previous studies have shown that CD40/CD40L interactions modulate the strength of interactions between GC B cells and T follicular helper (Tfh) cells, as well as subsequent B cell selection and differentiation. CD40 signaling in the presence of IL-21 induces c-Myc, which is essential for modulating Tfh cell engagement and promoting positive selection of GC B cells. c-Myc is a target of GSK3, and here the authors show that inactivation of GSK3 induces c-Myc and Foxo1 expression upon CD40/IL-21 stimulation of GC B cells, which drives expression of additional transcription factors, including IRF4, which are needed for PC differentiation and fate commitment. GSK3 inactivation and increased Foxo1 expression was also associated with facilitating the light zone to dark zone transition of GC B cells, another feature of PC fate commitment. Together, these results define a role for GSK3 as a master regulator of GC B cell differentiation and commitment.

Fibrosis typically occurs in response to allergic, autoimmune, and infectious diseases, and has been linked to TH2 and IgG4 responses. In this Top Read, Allard-Chamard et al. (p. 524) now show that fibrosing mediastinitis (FM) resulting from Histoplasma capsulatum infection shares histological features with IgG4-related disease but is associated with the accumulation of CD4+CTLs in fibrotic lesions. Previous work from this group has shown that CD4+ CTLs expand clonally and accumulate in disease lesions for the human autoimmune disorders, IgG4-related disease, and systemic sclerosis. Here they observe that FM patients have clonally expanded effector CD4+ T cells in peripheral blood and these CD4+ CTLs can be found in greater number in disease lesions compared with TH2 cells. CD4+ CTLs from FM patients can be reactivated upon ex vivo exposure to H. capsulatum Ag, suggesting a role for this fungal pathogen in FM pathogenesis. These results indicate that CD4+ CTLs and not TH2 cells are likely to contribute to the pathogenesis of infection-associated fibrosis.

The urgency of the current COVID-19 pandemic has led to the development of new tools and protocols to study immune responses to SARS-CoV-2. In this Top Read, Zelba et al. (p. 580) have developed a protocol to detect SARS-CoV-2–reactive T cells in PBMCs from COVID-19 patients. They adapted an existing protocol designed to detect rare tumor neoantigen-specific memory T cells in cancer patients. Here they expanded CD4+ and CD8+ T cells in vitro for 6 d, then stimulated them for 12 h in the presence of overlapping SARS-CoV-2 peptide libraries. T cell responses were measured using intracellular staining for CD154, TNF, IL-2 and IFN-γ. CD4+ and CD8+ T cells with reactivity to SARS-CoV-2 spike protein were detected in all ten convalescent COVID-10 patient samples, but membrane and nucleocapsid-specific CD4+ T cells were detected in only a subset of patients. No SARS-CoV-2–specific T cells were detected in uninfected individuals using this protocol, but prolonged overnight stimulation with viral peptides did induce cytokine responses in these control samples. These results describe a highly specific tool for measuring SARS-CoV-2–specific T cells in PBMCs and contribute to our understanding of the specificity of these responses in convalescent patients.

Dendritic cells (DCs) require the expression of Ag-specific peptide–MHC class II (pMHC-II) complexes on the cell surface to function as Ag presenting cells that can activate Ag-specific CD4+ T cells. pMHC-II complex turnover decreases in activated DCs, which helps to increase the likelihood of APC–T cell interactions. This turnover process is regulated to ubiquitination of MHC-II by the E3 ubiquitin ligase March-I. Previous studies in March-I knockout (KO) mice have shown that pMHC-II molecules accumulate on the surface of resting DCs, and in this Top Read, Kim et al. (p. 494) show that dysregulation of MHC-II turnover alters DC development and function. DCs from MHC-II ubiquitination–deficient mice (March-I KO mice or MHC-II K225R mice) have high levels of pMHC-II surface expression but do not effectively stimulate CD4+ T cells. This defect was independent of Ag presentation and was cell intrinsic, but it was not due to defects in T or B cell interactions. Expression analysis of DCs from MHC-II K225R mice revealed downregulation of genes involved in cell activation, but stimulation of MHC-II K225R DCs with LPS reversed this defect and enabled DCs to activate T cells. Taken together, these findings show that DC dysfunction in MHC-II ubiquitination–deficient mice is caused by DC intrinsic changes in gene expression.