In This Issue

doi:10.4049/jimmunol.1690017

Neonatal T and B Cells Experience Dichotomous Tolerance to Retroviruses

A number of viruses that establish persistent infection can be transmitted from mother to child in utero, during birth, or after birth. Some viruses can chronically infect primary lymphoid organs and exploit clonal deletion of pathogen-specific lymphocytes, thereby establishing central tolerance to the pathogen. To date, the relationship between T and B cell central tolerance in neonatal infections is not entirely clear and in this issue, Mavrommatis et al. (p. 3628) investigate the induction of adaptive immunity to a neonatal viral infection under conditions leading to pathogen central tolerance. The authors established a mouse model of retroviral transmission in which a murine leukemia virus (MLV), RARV2, was transmitted vertically from Rag1^−/− dams to immunocompetent offspring. These studies were conducted on a C57BL/6 background, which carries a defective MLV provirus in the germline, rendering animals partially tolerant to MLV. In this model, both dams and offspring need to be Rag1-sufficient for the effective induction of an antiviral IgG response in the offspring, suggesting the cooperation between maternal and offspring immunity in preventing viral transmission. Infection of RARV2 neonatally infected WT mice with Friend Virus (FV), which shares 79% amino acid identity with the envelope glycoproteins of RARV2, induced a significant FV-specific antiviral Ab response, demonstrating that neonatal RARV2 infection did not compromise the ability to mount an immune response to a homologous virus. Maternal immunodeficiency appeared to compromise the function of RARV2-specific CD4⁺ T cell responses in offspring, as CD4⁺ T cells from MLV-specific TCRβ-transgenic pups neonatally infected with RARV2 displayed decreased activity toward MLV Ags compared with offspring of immunosufficient mothers. Additionally, neonatal infection of MLV-specific TCRαβ-transgenic pups from Rag1^−/− dams resulted in a complete lack of mature CD4⁺ thymocytes and splenocytes, suggesting that retroviral transmission prevented thymic development of virus-specific CD4⁺ T cells in these pups. Examination of the B cell populations in neonatally infected WT animals revealed that while RARV2-specific B cell responses were slightly impaired, maternal immunodeficiency did not ablate virus-specific B cell responses. Importantly, when T cells from uninfected MLV-specific TCRβ-transgenic donors were transferred into neonatally infected WT animals, RARV2-specific B cell responses were significantly increased and viral replication was controlled, indicating a key role for T cell help in effective neonatal antiviral B cell responses. Taken together, these data suggest that maternal immunodeficiency permitted neonatal central T cell tolerance to a vertically transmitted virus, while neonatal B cells escaped deletional tolerance and retained their ability to respond to the pathogen. Therefore, this study suggests that T cell immunotherapy may be used to restore B cell responses in chronic infections, such as HIV, in which CD4⁺ T cell help is known to be defective.

JAK Signaling Threshold Determines Cytokine Secretion in Macrophages

Inflammatory bowel disease (IBD) is characterized by dysregulated cytokine production, which is attributed to multiple IBD-risk loci within cytokine-associated pathways, including a risk locus within the JAK2 region. Given that JAK proteins are required for some cytokine receptor signaling pathways, determining the effects of the JAK2 risk locus on immune outcomes in IBD patients may elucidate why, despite proving effective for a number of immune-mediated diseases, JAK inhibitors have demonstrated little to no efficacy in IBD clinical trials. Innate immunity plays an important role in IBD disease progression, and JAK inhibition may impact inflammatory responses in myeloid cells differently than in other cell subsets. In this issue, Hedl et al. (p. 3695) investigated the role of JAK and IBD-associated JAK2 variants in regulating myeloid cell responses to microbial products. Knockdown of JAK proteins in human monocyte-derived macrophages (MDMs) demonstrated that the JAK pathway regulates NOD2-dependent secretion of pro- and anti-inflammatory cytokines and NOD2-induced tolerance in myeloid cells. Mechanistically, as determined by receptor knockdown and neutralizing Ab studies, this regulation appears to be dependent on the autocrine/paracrine action of inhibitory cytokines IL-4, IL-10, IL-22, and TSLP, suggesting a model in which JAKs are not only required for the production of inhibitory cytokines, but are also activated by them. Examination of human MDMs carrying the IBD-associated polymorphism in the JAK2 region revealed that when compared with normal controls, JAK2 expression was significantly increased, as were pro- and anti-inflammatory cytokine secretion following acute NOD2 stimulation. Interestingly, progressive knockdown of JAK2 expression in IBD-risk carrier MDMs demonstrated that the threshold of JAK2 expression and signaling determined NOD2-induced outcomes; while anti-inflammatory cytokine secretion decreased with progressive JAK2 knockdown, proinflammatory cytokines switched from decreased to increased secretion once knockdown of JAK2 fell below 30%. Taken together, these data indicate that following NOD2 stimulation, JAK2 expression levels determine whether proinflammatory cytokine secretion increases or decreases. These observations were confirmed by studies assessing the effects of JAK inhibitors, used in clinically relevant dosages, on acute pattern-recognition receptor responses in IBD-risk carrier MDMs. In conclusion, this study determines that the JAK pathway regulates both pro- and anti-inflammatory cytokine secretion in myeloid cells and reveals that cytokine secretion is impacted by the threshold of JAK signaling. As JAK signaling threshold is a critical determinant of the balance between the secretion of pro- and anti-inflammatory cytokines, this study demonstrates that consideration of the JAK2 genotype and cell-specific targeting may improve JAK inhibitor therapy in the context of IBD.

Gut Foxp3⁺RORγt⁺ T Cells: Origin Story

Several subsets of regulatory T cells (Tregs) have been defined based on their differential expression of transcription factors commonly associated with effector T cell subsets, potentially including a recently described Foxp3⁺RORγt⁺ cell subset enriched in the colonic lamina propria. These cells, which coexpress transcription factors important for both immunomodulatory Tregs and proinflammatory Th17 cells, appear to protect against mucosal autoimmunity. In this issue, Solomon and Hsieh (p. 3512) analyzed Foxp3⁺RORγt⁺ cells to better understand their relationship to conventional Tregs and Th17 cells. Comparing the TCRα repertoires of different colonic T cell subsets in transgenic mice with a fixed TCRβ-chain, the authors found that the diversity of the Foxp3⁺RORγt⁺ cell repertoire was somewhat limited and most similar to that of Th17 cells. Th17 and Foxp3⁺RORγt⁺ cells, but no other pairs of subsets analyzed, shared a few of their common TCR specificities, though in general each subset had a unique repertoire. To assess whether Foxp3⁺RORγt⁺ cells might represent a subset of either Th17 cells or Tregs, the authors used transgenic mice bearing CT2, the αβTCR that was most highly represented among Foxp3⁺RORγt⁺ cells. Transfer of naive T cells from these mice into wild-type hosts revealed that peripherally induced Foxp3 expression preceded RORγt expression in CT2 T cells, and the latter was expressed only in cells also expressing Foxp3. Similar results were observed in a polyclonal system, supporting the idea that Foxp3⁺RORγt⁺ cells develop in the lymphoid periphery via a Treg intermediate and are thus a subset of Tregs, not of Th17 cells. Analysis of dendritic cell requirements for Foxp3⁺RORγt⁺ cell development indicated that CX3CR1⁺ APCs, which are important for commensal bacteria-driven differentiation of RORγt-expressing Th17 cells, were also involved in Foxp3⁺RORγt⁺ cell differentiation. These data advance our understanding of the origin of the Foxp3⁺RORγt⁺ cell subset, but additional work will be necessary to fully dissect its function.