CD4+ T Cell Differentiation in Chronic Viral Infections: The Tfh Perspective

Abstract

CD4⁺ T cells play a critical role in the response to chronic viral infections during the acute phase and in the partial containment of infections once chronic infection is established. As infection persists, the virus-specific CD4⁺ T cell response begins to shift in phenotype. The predominant change described in both mouse and human studies of chronic viral infection is a decrease in detectable T helper type (Th)1 responses. Some Th1 loss is due to decreased proliferative potential and decreased cytokine production in the setting of chronic antigen exposure. However, recent data suggest that Th1 dysfunction is accompanied by a shift in the differentiation pathway of virus-specific CD4⁺ T cells, with enrichment for cells with a T follicular helper cell (Tfh) phenotype. A Tfh-like program during chronic infection has now been identified in virus-specific CD8⁺ T cells as well. In this review, we discuss what is known about CD4⁺ T cell differentiation in chronic viral infections, with a focus on the emergence of the Tfh program and the implications of this shift with respect to Tfh function and the host-pathogen interaction.

Figure 1. Tfh cell differentiation during acute and chronic viral infections

Commitment to the Tfh lineage after an acute viral infection is a multi-step response requiring both priming by antigen presenting cells (APCs) and additional interactions with B cells. Tfh cell differentiation is further informed by a balance of cytokine, costimulatory, and inhibitory signals that allow the Tfh cell program to mature. Following differentiation, the function of Tfh cells is informed by anatomic location and by the tight coordination of Tfh cell interactions with B cells within the follicle and germinal center. These spatial interactions are guided by the lymphoid architecture. Chronic viral infections are associated with increased Tfh frequencies, increased Tfh-like transcriptional programs in CD8⁺ T cells, as well as by a shift from high quality antibody production, to lower quality, albeit higher quantity of antibody production. The mechanisms that drive this process remain incompletely understood, but continued TCR stimulation and interferon (IFN) signaling due to chronic viral infection presumably affect many components of Tfh cell differentiation and function, including disruption of the lymphoid architecture; changes in the polarization of the APC; direct effects on B cells and germinal center (GC) B cells; alterations in the cytokine milieu; and changes in costimulatory and inhibitory molecule expression as a result of persistent TCR stimulation.

Figure 2. Limitations of the most commonly used assays to characterize CD4⁺ T cell responses in humans

Virus-specific CD4⁺ T cells are heterogeneous, and assays that rely on cytokine production alone will inherently miss the breadth of specific cells. This poor detection is especially true in chronic viral infections, where T cells can exhibit impaired cytokine production (Exhausted Th1 cells, Th1-Ex). Use of in vitro stimulation and assessment cytokine production by multiplex immunoassay allows for a diverse capture of secreted cytokines but does not detect cells with minimal cytokine secretion and this approach is unable to identify the sources of expression and co-expression. Flow cytometry after in vitro stimulation allows for resolution of individual cells but is limited to cells that produce robust quantities of cytokines. Recent studies have used activation induced markers to identify activated cells that do not produce readily detectable cytokine upon TCR stimulation. HLA class II tetramers allow for the isolation of all CD4⁺ T cells with specificity for a given epitope, regardless of whether the cells can proliferate or produce cytokines. Transcriptional and epigenetic analyses of these cells gives a broad understanding of their phenotypes, although the phenotype of identified cells is likely biased by the epitope. Further, bulk transcriptional analyses of even tetramer-sorted cells are still limited by the inability to resolve whether transcriptional programs are expressed in separate cell populations or whether genes are co-expressed in the same cells. Single cell approaches such as flow cytometry, mass cytometry, and single cell sequencing allow for resolution of such true heterogeneity.