Effector-Phase IL-2 Signals Drive Th1 Effector and Memory Responses Dependently and Independently of TCF-1

Following viral infection, CD4+ T cell differentiation is tightly regulated by cytokines and TCR signals. Although most activated CD4+ T cells express IL-2Rα after lymphocytic choriomeningtis virus infection, by day 3 postinfection, only half of activated T cells maintain expression. IL-2Rα at this time point distinguishes precursors for terminally differentiated Th1 cells (IL-2Rαhi) from precursors for Tfh cells and memory T cells (IL-2Rαlo) and is linked to strong TCR signals. In this study, we test whether TCR-dependent IL-2 links the TCR to CD4+ T cell differentiation. We employ a mixture of anti-IL-2 Abs to neutralize IL-2 throughout the primary CD4+ T cell response to lymphocytic choriomeningitis virus infection in mice or only after the establishment of lineage-committed effector cells (day 3 postinfection). We report that IL-2 signals drive the formation of Th1 precursor cells in the early stages of the immune response and sustain Th1 responses during its later stages (after day 3). Effector-stage IL-2 also shapes the composition and function of resulting CD4+ memory T cells. Although IL-2 has been shown previously to drive Th1 differentiation by reducing the activity of the transcriptional repressor TCF-1, we found that sustained IL-2 signals were still required to drive optimal Th1 differentiation even in the absence of TCF-1. Therefore, we concluded that IL-2 plays a central role throughout the effector phase in regulating the balance between Th1 and Tfh effector and memory cells via mechanisms that are both dependent and independent of its role in modulating TCF-1 activity.

Tet2 deletion in CD4+ T cells disrupts Th1 lineage commitment in memory cells and enhances T follicular helper cell recall responses to viral rechallenge

Following viral clearance, antigen-specific CD4+ T cells contract and form a pool of distinct Th1 and Tfh memory cells that possess unique epigenetic programs, allowing them to rapidly recall their specific effector functions upon rechallenge. DNA methylation programing mediated by the methylcytosine dioxygenase Tet2 contributes to balancing Th1 and Tfh cell differentiation during acute viral infection; however, the role of Tet2 in CD4+ T cell memory formation and recall is unclear. Using adoptive transfer models of antigen-specific wild type and Tet2 knockout CD4+ T cells, we find that Tet2 is required for full commitment of CD4+ T cells to the Th1 lineage and that in the absence of Tet2, memory cells preferentially recall a Tfh like phenotype with enhanced expansion upon secondary challenge. These findings demonstrate an important role for Tet2 in enforcing lineage commitment and programing proliferation potential, and highlight the potential of targeting epigenetic programing to enhance adaptive immune responses.

Generation of antigen-specific memory CD4 T cells by heterologous immunization enhances the magnitude of the germinal center response upon influenza infection

Current influenza vaccine strategies have yet to overcome significant obstacles, including rapid antigenic drift of seasonal influenza viruses, in generating efficacious long-term humoral immunity. Due to the necessity of germinal center formation in generating long-lived high affinity antibodies, the germinal center has increasingly become a target for the development of novel or improvement of less-efficacious vaccines. However, there remains a major gap in current influenza research to effectively target T follicular helper cells during vaccination to alter the germinal center reaction. In this study, we used a heterologous infection or immunization priming strategy to seed an antigen-specific memory CD4+ T cell pool prior to influenza infection in mice to evaluate the effect of recalled memory T follicular helper cells in increased help to influenza-specific primary B cells and enhanced generation of neutralizing antibodies. We found that heterologous priming with intranasal infection with acute lymphocytic choriomeningitis virus (LCMV) or intramuscular immunization with adjuvanted recombinant LCMV glycoprotein induced increased antigen-specific effector CD4+ T and B cellular responses following infection with a recombinant influenza strain that expresses LCMV glycoprotein. Heterologously primed mice had increased expansion of secondary Th1 and Tfh cell subsets, including increased CD4+ TRM cells in the lung. However, the early enhancement of the germinal center cellular response following influenza infection did not impact influenza-specific antibody generation or B cell repertoires compared to primary influenza infection. Overall, our study suggests that while heterologous infection/immunization priming of CD4+ T cells is able to enhance the early germinal center reaction, further studies to understand how to target the germinal center and CD4+ T cells specifically to increase long-lived antiviral humoral immunity are needed.

Immunogen-Specific Strengths and Limitations of the Activation-Induced Marker Assay for Assessing Murine Antigen-Specific CD4+ T Cell Responses

The activation-induced marker (AIM) assay is a cytokine-independent technique to identify Ag-specific T cells based on the upregulated expression of activation markers after Ag restimulation. The method offers an alternative to intracellular cytokine staining in immunological studies, in which limited cytokine production makes the cell subsets of interest difficult to detect. Studies of lymphocytes in human and nonhuman primates have used the AIM assay to detect Ag-specific CD4+ and CD8+ T cells. However, there is a lack of validation of the strengths and limitations of the assay in murine (Mus musculus) models of infection and vaccination. In this study, we analyzed immune responses of TCR-transgenic CD4+ T cells, including lymphocytic choriomeningitis virus-specific SMARTA, OVA-specific OT-II, and diabetogenic BDC2.5-transgenic T cells, and measured the ability of the AIM assay to effectively identify these cells to upregulate AIM markers OX40 and CD25 following culture with cognate Ag. Our findings indicate that the AIM assay is effective for identifying the relative frequency of protein immunization-induced effector and memory CD4+ T cells, whereas the AIM assay had reduced ability to identify specific cells induced by viral infection, particularly during chronic lymphocytic choriomeningitis virus infection. Evaluation of polyclonal CD4+ T cell responses to acute viral infection demonstrated that the AIM assay can detect a proportion of both high- and low-affinity cells. Together, our findings indicate that the AIM assay can be an effective tool for relative quantification of murine Ag-specific CD4+ T cells to protein vaccination, while demonstrating its limitations during conditions of acute and chronic infection.

Tet2 coordinates with Foxo1 and Runx1 to balance T follicular helper cell and T helper 1 cell differentiation

In response to various types of infection, naïve CD4+ T cells differentiate into diverse helper T cell subsets; however, the epigenetic programs that regulate differentiation in response to viral infection remain poorly understood. Demethylation of CpG dinucleotides by Tet methylcytosine dioxygenases is a key component of epigenetic programing that promotes specific gene expression, cellular differentiation, and function. We report that following viral infection, Tet2-deficient CD4+ T cells preferentially differentiate into highly functional germinal center T follicular helper (TFH) cells that provide enhanced help for B cells. Using genome-wide DNA methylation and transcription factor binding analyses, we find that Tet2 coordinates with multiple transcription factors, including Foxo1 and Runx1, to mediate the demethylation and expression of target genes, including genes encoding repressors of TFH differentiation. Our findings establish Tet2 as an important regulator of TFH cell differentiation and reveal pathways that could be targeted to enhance immune responses against infectious disease.

Tet2-mediated programing balances T follicular helper cell and T helper 1 cell differentiation

The differentiation of CD4+ T cell subsets in response to infection has been studied extensively, however the epigenetic programs that regulate these processes remain poorly understood. Active demethylation by tet methylcytosine dioxygenases of CpG dinucleotides within DNA is a key component of epigenetic programing that promotes lineage specific gene expression and contributes to cellular differentiation and function. Here we report that Tet2 acts to restrict the differentiation of T follicular helper (Tfh) cells in CD4+ T cells responding to viral infection. Using an adoptive transfer model of virus-specific CD4+ cells we found that Tet2-deficient CD4+ T cells skew away from the Th1 lineage and instead preferentially differentiate into highly functional germinal center (GC) Tfh cells that provide enhanced help for B cell responses. We found that the impact of Tet2-mediated programing on CD4+ T cell differentiation is cell intrinsic and the shift in lineage differentiation occurs as early as 2 days post infection. Using genome-wide expression, DNA methylation and transcription factor binding analyses, we found that Tet2 coordinates with multiple transcription factors to mediate the demethylation and expression of their target genes following activation. Our findings establish Tet2 as an important regulator of Tfh cell differentiation and reveal pathways that could be targeted to enhance GC responses against infectious disease.

Supported by grants from NIH (R01 AI137238 to J.S.H., T32 AI055434 to L.M.S., and T32 AI138945 to A.B.

Protein Immunization Induces Memory CD4+ T Cells That Lack Th Lineage Commitment

Acute viral infection generates lineage-committed Th1 and T follicular helper (Tfh) memory cells that recall their lineage-specific functions following secondary challenge with virus. However, the lineage commitment of effector and memory Th cells in vivo following protein vaccination is poorly understood. In this study, we analyzed effector and memory CD4⁺ T cell differentiation in mice (Mus musculus) following adjuvanted glycoprotein immunization compared with acute lymphocytic choriomeningitis virus infection. Glycoprotein immunization induced CXCR5^- non-Tfh effector and memory CD4⁺ T cells that surprisingly had not undergone polarization toward any particular Th cell lineage but had undergone memory differentiation. However, upon challenge with virus, these Th lineage-nonpolarized memory CD4⁺ T cells were able to generate Th1 secondary effector cells, demonstrating their lineage plasticity. In addition, Tfh and memory Tfh cells were generated in response to protein immunization, and these cells differed from infection-induced Tfh cells by their lack of the transcription factor Tbet. Rechallenge experiments demonstrated that viral infection, but not protein immunization, during either the primary or secondary immune response, restricts the recall of Bcl6 expression and the generation of germinal center Tfh cells. Together, these data demonstrate that protein immunization generates a combination of nonpolarized memory cells that are highly plastic and memory Tfh cells that can undergo further Th1-like modulation during a secondary response to viral infection.

Protein immunization induces T follicular helper memory cells and nonpolarized T helper (Tnp) memory cells

Acute viral infection generates lineage-committed T helper 1 (Th1) and T follicular helper (Tfh) memory cells that recall their lineage-specific functions following secondary challenge with virus. However, the lineage commitment of effector and memory T helper cells in vivo following protein vaccination is poorly understood. In this study, we analyzed effector and memory CD4+ T cell differentiation following adjuvanted glycoprotein immunization compared to acute lymphocytic choriomeningitis virus (LCMV) infection. Glycoprotein immunization-induced CXCR5-non-Tfh effector and memory CD4+ T cells that surprisingly had not undergone polarization toward any particular T helper cell lineage but had undergone memory differentiation. However, upon challenge with virus, these T helper lineage-nonpolarized memory CD4+ T cells were able to generate Th1 secondary effector cells, demonstrating their lineage plasticity. In addition, Tfh and memory Tfh cells were generated in response to protein immunization, and these cells differed from infection-induced Tfh cells by their lack of the transcription factor Tbet. Rechallenge experiments demonstrated that viral infection, but not protein immunization, during either the primary or secondary immune response, restricts the recall of Bcl6 expression and the generation of germinal center Tfh cells. Together, these data demonstrate that protein immunization generates a combination of nonpolarized memory cells that are highly plastic and memory Tfh cells that can undergo further Th1-like modulation during a secondary response to viral infection.

Intracellular Cryptococcus neoformans disrupts the transcriptome profile of M1- and M2-polarized host macrophages

Macrophages serve as a first line of defense against infection with the facultative intracellular pathogen, Cryptococcus neoformans (Cn). However, the ability of these innate phagocytic cells to destroy ingested Cn is strongly influenced by polarization state with classically (M1) activated macrophages better able to control cryptococcal infections than alternatively (M2) activated cells. While earlier studies have demonstrated that intracellular Cn minimally affects the expression of M1 and M2 markers, the impact on the broader transcriptome associated with these states remains unclear. To investigate this, an in vitro cell culture model of intracellular infection together with RNA sequencing-based transcriptome profiling was used to measure the impact of Cn infection on gene expression in both polarization states. The gene expression profile of both M1 and M2 cells was extensively altered to become more like naive (M0) macrophages. Gene ontology analysis suggested that this involved changes in the activity of the Janus kinase-signal transducers and activators of transcription (JAK-STAT), p53, and nuclear factor-κB (NF-κB) pathways. Analyses of the principle polarization markers at the protein-level also revealed discrepancies between the RNA- and protein-level responses. In contrast to earlier studies, intracellular Cn was found to increase protein levels of the M1 marker iNos. In addition, common gene expression changes were identified that occurred post-Cn infection, independent of polarization state. This included upregulation of the transcriptional co-regulator Cited1, which was also apparent at the protein level in M1-polarized macrophages. These changes constitute a transcriptional signature of macrophage Cn infection and provide new insights into how Cn impacts gene expression and the phenotype of host phagocytes.

Modulation of Macrophage Inflammatory Nuclear Factor κB (NF-κB) Signaling by Intracellular Cryptococcus neoformans

Cryptococcus neoformans (Cn) is a common facultative intracellular pathogen that can cause life-threatening fungal meningitis in immunocompromised individuals. Shortly after infection, Cn is detectable as both extra- and intracellular yeast particles, with Cn being capable of establishing long-lasting latent infections within host macrophages. Although recent studies have shown that shed capsular polysaccharides and intact extracellular Cn can compromise macrophage function through modulation of NF-κB signaling, it is currently unclear whether intracellular Cn also affects NF-κB signaling. Utilizing live cell imaging and computational modeling, we find that extra- and intracellular Cn support distinct modes of NF-κB signaling in cultured murine macrophages. Specifically, in RAW 264.7 murine macrophages treated with extracellular glucuronoxylomannan (GXM), the major Cn capsular polysaccharide, LPS-induced nuclear translocation of p65 is inhibited, whereas in cells with intracellular Cn, LPS-induced nuclear translocation of p65 is both amplified and sustained. Mathematical simulations and quantification of nascent protein expression indicate that this is a possible consequence of Cn-induced "translational interference," impeding IκBα resynthesis. We also show that long term Cn infection induces stable nuclear localization of p65 and IκBα proteins in the absence of additional pro-inflammatory stimuli. In this case, nuclear localization of p65 is not accompanied by TNFα or inducible NOS (iNOS) expression. These results demonstrate that capsular polysaccharides and intact intracellular yeast manipulate NF-κB via multiple distinct mechanisms and provide new insights into how Cn might modulate cellular signaling at different stages of an infection.