Helminth-induced Th2 cell dysfunction is distinct from exhaustion and is maintained in the absence of antigen

Abstract

T cell-intrinsic regulation, such as anergy, adaptive tolerance and exhaustion, is central to immune regulation. In contrast to Type 1 and Type 17 settings, knowledge of the intrinsic fate and function of Th2 cells in chronic Type 2 immune responses is lacking. We previously showed that Th2 cells develop a PD-1/PD-L2-dependent intrinsically hypo-responsive phenotype during infection with the filarial nematode Litomosoides sigmodontis, denoted by impaired functionality and parasite killing. This study aimed to elucidate the transcriptional changes underlying Th2 cell-intrinsic hypo-responsiveness, and whether it represents a unique and stable state of Th2 cell differentiation. We demonstrated that intrinsically hypo-responsive Th2 cells isolated from L. sigmodontis infected mice stably retained their dysfunctional Th2 phenotype upon transfer to naïve recipients, and had a divergent transcriptional profile to classical Th2 cells isolated prior to hypo-responsiveness and from mice exposed to acute Type 2 stimuli. Hypo-responsive Th2 cells displayed a distinct transcriptional profile to exhausted CD4 ⁺ T cells, but upregulated Blimp-1 and the anergy/regulatory-associated transcription factors Egr2 and c-Maf, and shared characteristics with tolerised T cells. Hypo-responsive Th2 cells increased mRNA expression of the soluble regulatory factors Fgl2, Cd38, Spp1, Areg, Metrnl, Lgals3, and Csf1, and a subset developed a T-bet ⁺IFN-γ ⁺ Th2/Th1 hybrid phenotype, indicating that they were not functionally inert. Contrasting with their lost ability to produce Th2 cytokines, hypo-responsive Th2 cells gained IL-21 production and IL-21R blockade enhanced resistance to L. sigmodontis. IL-21R blockade also increased the proportion of CD19 ⁺PNA ⁺ germinal centre B cells and serum levels of parasite specific IgG1. This indicates a novel regulatory role for IL-21 during filarial infection, both in controlling protection and B cell responses. Thus, Th2 cell-intrinsic hypo-responsiveness is a distinct and stable state of Th2 cell differentiation associated with a switch from a classically active IL-4 ⁺IL-5 ⁺ Th2 phenotype, to a non-classical dysfunctional and potentially regulatory IL-21 ⁺Egr2 ⁺c-Maf ⁺Blimp-1 ⁺IL-4 ^loIL-5 ^loT-bet ⁺IFN-γ ⁺ Th2 phenotype. This divergence towards alternate Th2 phenotypes during chronicity has broad implications for the outcomes and treatment of chronic Type 2-related infections and diseases.

Author summary

Helminth parasites chronically infect over 1 billion people, with filarial nematodes accounting for 120 million. Protection is mediated by Th2 cells, but infection invokes dominant down-regulatory immune responses forming a major barrier to the development of protective immunity. We previously demonstrated that during murine filarial infection Th2 cells change phenotype, developing a novel form of Th2 cell-intrinsic dysfunction that impairs parasite clearance. Here we investigate the gene expression profile of Th2 cell dysfunction, and whether it represents a unique and stable form of T cell differentiation. Dysfunctional Th2 cells had a different gene expression profile to effector Th2 cells, and retained their unresponsive phenotype in the absence of antigen and active infection when transferred to a naïve recipient. Dysfunctional Th2 cells produced IL-21, through which they may inhibit protective immunity, as IL-21R neutralisation increased parasite killing. In comparison to known forms of T cell intrinsic regulation, dysfunctional Th2 cells had a distinct gene expression profile to exhausted T cells, but shared characteristics with anergy and tolerance. Together this indicates that Th2 cell dysfunction represents a unique and stable state of Th2 cell differentiation, which has important implications for understanding the outcomes of helminth infections and designing therapies for Th2-mediated allergies.