PTPN2-deficiency exacerbates T follicular helper cell and B cell responses and promotes the development of autoimmunity

Highlights

• Hematopoietic PTPN2-deficiency promotes systemic inflammation and autoimmunity.

• PTPN2-deficiency enhances T cell and B cell responses.

• PTPN2 deficiency increases T_fh cell generation.

• PTPN2 deficiency promotes IL-21-induced STAT3 signalling in B cells and T_fh cells.

Abstract

Non-coding single nucleotide polymorphisms that repress PTPN2 expression have been linked with the development of type 1 diabetes, rheumatoid arthritis and Crohn's disease. PTPN2 attenuates CD8⁺ T cell responses to self and prevents overt autoreactivity in the context of T cell homeostasis and antigen cross-presentation. The role of PTPN2 in other immune subsets in the development of autoimmunity remains unclear. Here we show that the inducible deletion of PTPN2 in hematopoietic compartment of adult non-autoimmune prone mice results in systemic inflammation and autoimmunity. PTPN2-deficient mice had increased inflammatory monocytes, B cells and effector T cells in lymphoid and non-lymphoid tissues and exhibited symptoms of dermatitis, glomerulonephritis, pancreatitis and overt liver disease. Autoimmunity was characterised by the formation of germinal centers in the spleen and associated with markedly increased germinal center B cells and T follicular helper (T_fh) cells and circulating anti-nuclear antibodies, inflammatory cytokines and immunoglobulins. CD8⁺ T cell proliferative responses were enhanced, and interleukin-21-induced STAT-3 signalling in T_fh cells and B cells was increased and accompanied by enhanced B cell proliferation ex vivo. These results indicate that deficiencies in PTPN2 across multiple immune lineages, including naive T cells, T_fh cells and B cells, contribute to the development of autoimmunity.

Introduction

The pathogenesis of autoimmune and inflammatory disorders is not fully understood, but commonly involves different genetic susceptibility genes and contributions from environmental factors that promote abnormal immune responses. Beyond the well-established associations of major histocompatibility complex (MHC) haplotypes with autoimmunity, genetic traits contributing to the development of complex diseases such as type 1 diabetes, rheumatoid arthritis, systemic lupus erythematosus and Crohn's disease have been extensively defined by genome-wide association (GWA) [1], [2], [3]. Many of the associated genes encode proteins involved in innate and adaptive immunity and the establishment and maintenance of immunological tolerance. These include protein tyrosine phosphatases (PTPs) that regulate T cell and/or B cell development or function [3], [4].

GWA studies have identified PTPN2 on chromosome 18, encoding PTPN2, also known as T cell PTP, as an important non-MHC locus gene for autoimmune and inflammatory diseases [3], [5], [6], [7], [8]. Non-coding PTPN2 single nucleotide polymorphisms (SNPs) have been linked with the development of Crohn's, rheumatoid arthritis, and type 1 diabetes [3], [5], [6], [7], [8]. Although it is not clear how these SNPs impact on PTPN2 mRNA and protein, at least one PTPN2 SNP (rs1893217) associated with type 1 diabetes results in a 40% decrease in PTPN2 mRNA in primary human CD4⁺CD25⁺ T_reg and CD4⁺CD45RO⁺ memory T cells [9]. Yet another PTPN2 SNP (rs2542151), associated with Crohn's disease, rheumatoid arthritis and type 1 diabetes [3] results in decreased PTPN2 protein in colonic lamina propria fibroblasts from Crohn's patients [10].

PTPN2 in an important negative regulator of cytokine signalling dephosphorylating and inactivating Janus-activated kinases (JAK)-1 and -3 and the JAK substrates, signal transducer and activators of transcription (STATs)-1, -3 and -5, downstream of cytokines such as interleukin (IL)-2, -7 and -6 and interferon (IFN)-γ [11], [12]. In keeping with PTPN2's ability to attenuate JAK/STAT signalling, mice that are globally deficient for PTPN2 (Ptpn2^−/−) die within 3–5 weeks from severe inflammatory disease accompanied by lymphocytic infiltrates in multiple organs and the increased tissue expression of pro-inflammatory cytokines such as TNF and IFN-γ [13], [14]. Ptpn2^−/− mice also exhibit perturbations in hematopoiesis and in particular defects in bone marrow B cell and erythroid development [13], [15]. Interestingly, bone marrow reconstitution studies have indicated that the hematopoietic defects and the overt morbidity and mortality in Ptpn2^−/− mice are dependent on the bone marrow stroma [13].

On the other hand, studies from our laboratory have shown that the inflammatory phenotype in Ptpn2−/− mice may be attributed at least in part to perturbations in T cell tolerance [16], [17], [18]. PTPN2 negatively regulates T cell receptor (TCR) signalling and sets the threshold for TCR-instigated responses in naive T cells by dephosphorylating and inactivating the Src family kinases Lck and Fyn [16]. We have shown that PTPN2 ‘tunes’ TCR signalling and T cell responses to self-peptide-MHC to prevent inappropriate T cell activation and thereby prevent the development of autoimmunity [14], [16], [17], [18]. T cell-specific PTPN2-deficient mice accumulate terminally differentiated effector/memory T cells with age and develop widespread inflammation and spontaneous autoimmunity [16]. Moreover, PTPN2-deficiency dramatically enhances TCR-dependent naive T cell proliferation in lymphopenic hosts, alters the TCR repertoire and results in the generation of antigen-experienced effector T cells, so that the adoptive transfer of naive homozygous or heterozygous PTPN2-deficient CD8⁺ T cells into sublethally irradiated lymphopenic hosts promotes tissue damage and autoimmunity [17]. In other studies we have shown that homozygous or heterozygous PTPN2 deficiency in CD8⁺ T cells can negate the need for CD4⁺ T cell help during antigen cross-presentation, so that PTPN2-deficiency permits the helper-independent acquisition of cytotoxic activity to self-antigens and the development of autoimmunity [18]. These studies have provided insight into the cellular and molecular mechanisms by which PTPN2 SNPs and PTPN2-deficiency may contribute to the development of inflammation and autoimmunity. However, the extent to which PTPN2-deficiency in the bone marrow stroma and in other immune and non-immune cells influences inflammation and autoimmunity remains unclear. In this study we have inducibly deleted PTPN2 in adult mice and assessed the contribution of the immune compartment to the development of inflammation and autoimmunity. We report that hematopoietic PTPN2 deficiency promotes systemic inflammation and overt autoimmunity accompanied by alterations in the lymphoid and myeloid compartments and enhanced T cell and B cell responses.