Julia Zeiträg

T follicular helper (T _FH ) cells are essential for effective antibody responses, but deciphering the intrinsic wiring of mouse T _FH cells has long been hampered by the lack of a reliable protocol for their generation in vitro. We report that transforming growth factor–β (TGF-β) induces robust expression of T _FH hallmark molecules CXCR5 and Bcl6 in activated mouse CD4 ⁺ T cells in vitro. TGF-β–induced mouse CXCR5 ⁺ T _FH cells are phenotypically, transcriptionally, and functionally similar to in vivo–generated T _FH cells and provide critical help to B cells. The study further reveals that TGF-β–induced CXCR5 expression is independent of Bcl6 but requires the transcription factor c-Maf. Classical TGF-β–containing T helper 17 (T _H 17)–inducing conditions also yield separate CXCR5 ⁺ and IL-17A–producing cells, highlighting shared and distinct cell fate trajectories of T _FH and T _H 17 cells. We demonstrate that excess IL-2 in high-density T cell cultures interferes with the TGF-β–induced T _FH cell program, that T _FH and T _H 17 cells share a common developmental stage, and that c-Maf acts as a switch factor for T _FH versus T _H 17 cell fates in TGF-β–rich environments in vitro and in vivo.

Constitutive T cell-intrinsic miRNA expression is required for the differentiation of naive CD4(+) T cells into Tfh cells, thus making it difficult to study the role of miRNAs in the maintenance of already established Tfh cells and ongoing germinal center (GC) responses. To overcome this problem, we here used temporally controlled ablation of mature miRNAs specifically in CD4(+) T cells during acute LCMV infection in mice. T cell-intrinsic miRNA expression was not only critical at early stages of Tfh cell differentiation, but also important for the maintenance of already established Tfh cells. In addition, CD4(+) T cell-specific ablation of miRNAs resulted in impaired GC B cell responses. Notably, miRNA deficiency also compromised the antigen-specific CD4(+) T cell compartment, Th1 cells, Treg cells, and Tfr cells. In conclusion, our results highlight miRNAs as important regulators of Tfh cells, thus providing novel insights into the molecular events that govern T cell-B cell interactions and Th cell identity.

Th cells integrate signals from their microenvironment to acquire distinct specialization programs for efficient clearance of diverse pathogens or for immunotolerance. Ionic signals have recently been demonstrated to affect T cell polarization and function. Sodium chloride (NaCl) was proposed to accumulate in peripheral tissues upon dietary intake and to promote autoimmunity via the Th17 cell axis. Here, we demonstrate that high-NaCl conditions induced a stable, pathogen-specific, antiinflammatory Th17 cell fate in human T cells in vitro. The p38/MAPK pathway, involving NFAT5 and SGK1, regulated FoxP3 and IL-17A expression in high-NaCl conditions. The NaCl-induced acquisition of an antiinflammatory Th17 cell fate was confirmed in vivo in an experimental autoimmune encephalomyelitis (EAE) mouse model, which demonstrated strongly reduced disease symptoms upon transfer of T cells polarized in high-NaCl conditions. However, NaCl was coopted to promote murine and human Th17 cell pathogenicity, if T cell stimulation occurred in a proinflammatory and TGF-β–low cytokine microenvironment. Taken together, our findings reveal a context-dependent, dichotomous role for NaCl in shaping Th17 cell pathogenicity. NaCl might therefore prove beneficial for the treatment of chronic inflammatory diseases in combination with cytokine-blocking drugs.

A growing body of evidence suggests that Cre recombinase can be toxic to immune cells in various experimental settings. Cre recombinase toxicity is dependent on the level of Cre activity and may also interfere with cell proliferation. Here, we compared two different published tamoxifen‐inducible CD4‐CreERT2 mouse lines for their suitability to study the dynamics of T‐follicular helper cell responses in vivo. Our data underscore that under certain circumstances inducible Cre toxicity (tamoxifen application results in translocation of preformed CreERT2 to the nucleus) interferes with cell survival and, therefore, necessitates careful interpretation of experimental data and the inclusion of appropriate controls. Interestingly, our data indicate that low expression of CreERT2 can still allow for efficient recombination in proliferating lymphocytes without causing excessive cell loss due to Cre toxicity.