T-Follicular Helper-like Cells in Sarcoidosis: Lending a Helping Hand.

T–follicular helper (Tfh) cells were first defined 12 years ago as a specialized subset of CD4+ T cells that expresses CXCR5, PD-1, and ICOS and the lineage-defining transcription factor, Bcl-6 (1–3). Tfh cells are strategically placed in lymphoid organs and are considered as nonmigratory compared with nonfollicular T-helper cells (4), which migrate from secondary lymphoid organs to nonlymphoid organs. In the past years, Tfh cells have gained attention as the major cell type regulating germinal center formation and B-cell antibody production and are involved in multiple immune disorders and infections (5). In this issue of the Journal, Bauer and colleagues (pp. 1403–1417) report the presence of Tfh-like cells in the BAL fluid of patients with sarcoidosis (6). These cells display features of Tfh cells while lacking CXCR5, which distinguishes Tfh cells from other T-cell subsets and is required for T-cell migration toward the T cell–B cell border of secondary lymphoid organs, and Bcl-6. In vitro, coculture of these Tfh-like cells with blood B cells induced B-cell proliferation and antibody production. This article thus adds an important role for Tfh-like cells in pulmonary sarcoidosis and potentially other immune-mediated lung disorders.

Sarcoidosis is a granulomatous lung disease that occurs in individuals of all ages, sexes, and ethnic backgrounds and is characterized by the accumulation of large numbers of activated CD4+ T cells and B cells in the lung (7). Although the cause of sarcoidosis remains unknown, a recent study identified a T-cell epitope derived from Aspergillus nidulans and suggested a potential role of this organism in driving Löfgren syndrome, an acute form of sarcoidosis (8). Multiple studies have delineated the phenotype of CD4+ T cells in the BAL fluid of patients with sarcoidosis, including the presence of CD4+ T cells expressing T-helper cell type 1 (Th1) and/or Th17 phenotypes as well as FoxP3-expressing regulatory T cells (9). Regulatory T cells have also been shown to be dysfunctional in sarcoidosis (10), and this dysfunctional state may contribute to the increased frequency of Th1- and/or Th17-polarized CD4+ T cells in the lung. A role for B cells also exists in sarcoidosis on the basis of the presence of increased IgA levels and the antivimentin antibodies (11). However, the subset of CD4+ T cells involved in helping B-cell differentiation and maintenance in sarcoidosis has not been defined.

Bauer and colleagues (6) sought to understand the function of pulmonary Tfh and germinal center–like lymphocytes in sarcoidosis and used flow cytometry to identify CXCR5, PD-1, ICOS, CD40L, and IL-21 expression on Tfh cells in the BAL fluid. Unfortunately, CD4+ T cells in the BAL fluid lacked expression of CXCR5 and Bcl-6, the two signature molecules that define Tfh cells. However, these cells expressed CD40L and IL-21 that promote B-cell expansion and plasma-cell differentiation. On the basis of CD40L and IL-21 expression, the authors named these cells as Tfh-like cells. Future studies assessing the presence of additional transcription factors, such as TCF-1 (T-cell factor 1) and the lymphoid enhancer–binding factor LEF1, in these cells will be useful to further characterize their Tfh-like status. Cytokine analysis of in vitro–stimulated CD4+ T cells revealed the presence of multiple subsets of T cells displaying Th1 (IFN-γ–secreting), Th17 (IL-17–secreting), and presumably Tfh-like phenotypes (IL-21) in the sarcoidosis BAL fluid. Secretion of IL-2, which is considered as a negative regulator of Tfh-cell differentiation (12), was also present. Therefore, future studies assessing the presence of CD4+ T cells that express either IL-21 alone or both IL-2 and IL-21 will surely add information about their role in sarcoidosis. Analysis of memory markers, CXCR3 and CD69, revealed a tissue-resident phenotype of these CD4+ T cells that was confirmed by RNA sequencing. Among the various tissues, BAL-fluid CD4+ T cells showed a dominant memory phenotype as compared with tonsils or blood.

Transcriptome analysis of BAL-fluid T cells was not of great help in assessing the relationship between Tfh-like cells and classical Tfh cells because both the surface marker and the lineage marker for Tfh cells, CXCR5 and Bcl-6, were absent. However, in vitro T cell–B cell coculture assays confirmed the ability of Tfh-like T cells in BAL fluid to induce B-cell plasmablast formation, similar to classical Tfh cells in the tonsils (13). In addition, in vitro IL-21–blocking experiments confirmed the ability of IL-21–producing Tfh-like cells to induce antibody production by B cells, indicating that these cells share functional homology with classical Tfh cells (Figure 1). Tfh cells contact B cells in organized structures present in the secondary lymphoid organs. In a nonlymphoid organ such as the lung, both sterile and pathogenic inflammation induce the formation of aggregates called ectopic lymphoid aggregates or tertiary lymphoid structures, which are comprised of T cells, B cells, and follicular dendritic cells (14). In ectopic lymphoid aggregates or loosely arranged aggregates, T cells exist in close contact with B cells, allowing T cell–B cell cooperation and T cell–mediated help to B cells. To find such aggregates and confirm the cooperation of Tfh-like cells with B cells, Bauer and colleagues (6) documented the close contact between Tfh-like cells and B cells. In contrast to ectopic lymphoid aggregates, most of the aggregates in the lungs of subjects with sarcoidosis were nonectopic (i.e., lacking follicular dendritic cells). The important highlight of this current manuscript is the presence of IL-21–producing Tfh-like cells in the lung of patients with sarcoidosis that are functionally similar to but phenotypically distinct from classical Tfh cells and provide necessary help to B cells to undergo class switching and formation of plasmablasts. Tfh-like cells have also been identified in the murine lung, playing a role in T cell–B cell cooperation (15). Future studies involving lineage tracing and gene depletion in mouse models will allow a further understanding the mechanism(s) involved in Tfh-cell differentiation and function that will significantly advance the field of Tfh-like cell biology. This study also raises the possibility of new therapeutic strategies targeting Tfh-like cells in modulating disease activity and the use of Tfh-like cells in blood as a biomarker of disease activity.

Figure 1.

IL-21–producing T–follicular helper (Tfh)-like cells promote B-cell differentiation and antibody (Ab) production in the lungs. In sarcoidosis, an unknown factor causes the recruitment of leukocytes that forms cellular aggregates in the peribronchovascular region of the lungs. Within these aggregates, PD-1+ICOS+CXCR5−Bcl6−CD4+ T cells, named as Tfh-like cells by Bauer and colleagues (6), presumably interacted with B cells. IL-21 secretion from Tfh-like cells (A) induces B-cell differentiation into plasmablasts and (B) is inhibited in the presence of blocking IL-21 Ab. (C) B-cell differentiation and Ab production are independent of CD40–CD40L interaction.