Alexandre Corthay

How do Regulatory T Cells Work

CD4+ T cells are commonly divided into regulatory T (Treg) cells and conventional T helper (Th) cells. Th cells control adaptive immunity against pathogens and cancer by activating other effector immune cells. Treg cells are defined as CD4+ T cells in charge of suppressing potentially deleterious activities of Th cells. This review briefly summarizes the current knowledge in the Treg field and defines some key questions that remain to be answered. Suggested functions for Treg cells include: prevention of autoimmune diseases by maintaining self‐tolerance; suppression of allergy, asthma and pathogen‐induced immunopathology; feto‐maternal tolerance; and oral tolerance. Identification of Treg cells remains problematic, because accumulating evidence suggests that all the presently‐used Treg markers (CD25, CTLA‐4, GITR, LAG‐3, CD127 and Foxp3) represent general T‐cell activation markers, rather than being truly Treg‐specific. Treg‐cell activation is antigen‐specific, which implies that suppressive activities of Treg cells are antigen‐dependent. It has been proposed that Treg cells would be self‐reactive, but extensive TCR repertoire analysis suggests that self‐reactivity may be the exception rather than the rule. The classification of Treg cells as a separate lineage remains controversial because the ability to suppress is not an exclusive Treg property. Suppressive activities attributed to Treg cells may in reality, at least in some experimental settings, be exerted by conventional Th cell subsets, such as Th1, Th2, Th17 and T follicular (Tfh) cells. Recent reports have also demonstrated that Foxp3+ Treg cells may differentiate in vivo into conventional effector Th cells, with or without concomitant downregulation of Foxp3.

Epitope glycosylation plays a critical role for T cell recognition of type II collagen in collagen-induced arthritis

Immunization of mice with type II collagen (CII) leads to collagen‐induced arthritis (CIA), a model for rheumatoid arthritis. T cell recognition of CII is believed to be a critical step in CIA development. We have analyzed the T cell determinants on CII and the TCR used for their recognition, using twenty‐nine T cell hybridomas derived from C3H.Q and DBA/1 mice immunized with rat CII. All hybridomas were specific for the CII(256 – 270) segment. However, posttranslational modifications (hydroxylation and variable O‐linked glycosylation) of the lysine at position 264 generated five T cell determinants that were specifically recognized by different T cell hybridoma subsets. TCR sequencing indicated that each of the five T cell epitopes selected its own TCR repertoire. The physiological relevance of this observation was shown by in vivo antibody‐driven depletion of TCR Vα2‐positive T cells, which resulted in an inhibition of the T cell proliferative response in vitro towards the non‐modified CII(256 – 270), but not towards the glycosylated epitope. Most hybridomas (20/29) specifically recognized CII(256 – 270) glycosylated with a monosaccharide (β‐D‐galactopyranose). We conclude that this glycopeptide is immunodominant in CIA and that posttranslational modifications of CII create new T cell determinants that generate a diverse TCR repertoire.

Inflammation driven by tumour-specific Th1 cells protects against B-cell cancer

The immune system can both promote and suppress cancer. Chronic inflammation and proinflammatory cytokines such as interleukin (IL)-1 and IL-6 are considered to be tumour promoting. In contrast, the exact nature of protective antitumour immunity remains obscure. Here, we quantify locally secreted cytokines during primary immune responses against myeloma and B-cell lymphoma in mice. Strikingly, successful cancer immunosurveillance mediated by tumour-specific CD4+ T cells is consistently associated with elevated local levels of both proinflammatory (IL-1α, IL-1β and IL-6) and T helper 1 (Th1)-associated cytokines (interferon-γ (IFN-γ), IL-2 and IL-12). Cancer eradication is achieved by a collaboration between tumour-specific Th1 cells and tumour-infiltrating, antigen-presenting macrophages. Th1 cells induce secretion of IL-1β and IL-6 by macrophages. Th1-derived IFN-γ is shown to render macrophages directly cytotoxic to cancer cells, and to induce macrophages to secrete the angiostatic chemokines CXCL9/MIG and CXCL10/IP-10. Thus, inflammation, when driven by tumour-specific Th1 cells, may prevent rather than promote cancer.

A Three-cell Model for Activation of Naïve T Helper Cells

It is generally assumed that the activation of naïve T helper (Th) cells is the result of a two‐cell interaction between the Th cell and a dendritic cell (DC) and that three signals are required. Signal one or stimulation is the recognition by the T‐cell receptor (TCR) of antigenic peptides presented by major histocompatibility complex (MHC) class II molecules. Signal two or co‐stimulation is mainly provided by the triggering of CD28 on the T cell by CD80 and CD86 molecules on the DC. Signal three or polarization directs T‐cell differentiation into various effector phenotypes such as Th1 and Th2. Both signals, two and three, are often assumed to result from the binding of microbial products or endogenous molecular danger signals to germline‐encoded receptors such as toll‐like receptors (TLR) on the DC. However, recent data challenge this two‐cell model by revealing that Th1 polarization requires the presence of interferon‐γ (IFN‐γ) provided by a third cell. I propose here a three‐cell model for naïve Th‐cell activation. In this model, delivery of signal three by the DC is dependent on help provided by other innate immune cells such as NK cells, NK T cells, γδ T cells, mast cells, eosinophils and basophils. The rationale behind this model is that the innate immune system has been designed by evolution to select an appropriate class of immune response to protect the host.

Does the Immune System Naturally Protect Against Cancer

The importance of the immune system in conferring protection against pathogens like viruses, bacteria, and parasitic worms is well established. In contrast, there is a long-lasting debate on whether cancer prevention is a primary function of the immune system. The concept of immunological surveillance of cancer was developed by Lewis Thomas and Frank Macfarlane Burnet more than 50 years ago. We are still lacking convincing data illustrating immunological eradication of precancerous lesions in vivo. Here, I present eight types of evidence in support of the cancer immunosurveillance hypothesis. First, primary immunodeficiency in mice and humans is associated with increased cancer risk. Second, organ transplant recipients, who are treated with immunosuppressive drugs, are more prone to cancer development. Third, acquired immunodeficiency due to infection by human immunodeficiency virus (HIV-1) leads to elevated risk of cancer. Fourth, the quantity and quality of the immune cell infiltrate found in human primary tumors represent an independent prognostic factor for patient survival. Fifth, cancer cells harbor mutations in protein-coding genes that are specifically recognized by the adaptive immune system. Sixth, cancer cells selectively accumulate mutations to evade immune destruction (“immunoediting”). Seventh, lymphocytes bearing the NKG2D receptor are able to recognize and eliminate stressed premalignant cells. Eighth, a promising strategy to treat cancer consists in potentiating the naturally occurring immune response of the patient, through blockade of the immune checkpoint molecules CTLA-4, PD-1, or PD-L1. Thus, there are compelling pieces of evidence that a primary function of the immune system is to confer protection against cancer.

How Do CD4+ T Cells Detect and Eliminate Tumor Cells That Either Lack or Express MHC Class II Molecules?

CD4+ T cells contribute to tumor eradication, even in the absence of CD8+ T cells. Cytotoxic CD4+ T cells can directly kill MHC class II positive tumor cells. More surprisingly, CD4+ T cells can indirectly eliminate tumor cells that lack MHC class II expression. Here, we review the mechanisms of direct and indirect CD4+ T cell-mediated elimination of tumor cells. An emphasis is put on T cell receptor (TCR) transgenic models, where anti-tumor responses of naïve CD4+ T cells of defined specificity can be tracked. Some generalizations can tentatively be made. For both MHCIIPOS and MHCIINEG tumors, presentation of tumor-specific antigen by host antigen-presenting cells (APCs) appears to be required for CD4+ T cell priming. This has been extensively studied in a myeloma model (MOPC315), where host APCs in tumor-draining lymph nodes are primed with secreted tumor antigen. Upon antigen recognition, naïve CD4+ T cells differentiate into Th1 cells and migrate to the tumor. At the tumor site, the mechanisms for elimination of MHCIIPOS and MHCIINEG tumor cells differ. In a TCR-transgenic B16 melanoma model, MHCIIPOS melanoma cells are directly killed by cytotoxic CD4+ T cells in a perforin/granzyme B-dependent manner. By contrast, MHCIINEG myeloma cells are killed by IFN-γ stimulated M1-like macrophages. In summary, while the priming phase of CD4+ T cells appears similar for MHCIIPOS and MHCIINEG tumors, the killing mechanisms are different. Unresolved issues and directions for future research are addressed.

T lymphocytes are not required for the spontaneous development of entheseal ossification leading to marginal ankylosis in the DBA/1 mouse

OBJECTIVE Male mice of the DBA/1 inbred strain spontaneously develop polyarthritis and toe stiffness when they are > or =4 months old. The arthritis affects predominantly the proximal interphalangeal joints and the ankle of the hind limbs. The current study was aimed at determining the importance of T lymphocytes in this disease. METHODS Histologic sections of hindpaws from arthritic DBA/1 mice were examined. The role of T lymphocytes was studied by using mice lacking either alpha/beta or gamma/delta T cells due to a deletion in T cell receptor beta (TCRbeta) or TCRdelta genes. RESULTS Arthritis was associated with a massive proliferation of connective tissue (fibroblasts) in synovium and adjacent tissues. Chondroid and bone tissue outgrowth at the entheses generated periarticular osteophytes (enthesophytes) which were deposited on the unchanged margins of the preexisting bone. In some cases, the enthesophytes enlarged enough to bridge and fuse the bones by marginal ankylosis. Articular cartilage was essentially unaffected. Abnormal chondroid tissue formation was common in stiffened toes, suggesting that the same pathology may underlie both joint stiffness and arthritis. Dividing chondrocytes were commonly seen in tendons, but without correlation with arthritis or toe stiffness. Mice lacking alpha/beta or gamma/delta T cells developed arthritis at the same incidence as control littermates. CONCLUSION The naturally occurring arthritis in male DBA/1 mice is a T cell-independent enthesopathy characterized by periarticular hyperostosis and marginal ankylosis. This suggests that the ossification leading to peripheral ankylosis of the joints in human enthesopathies, such as diffuse idiopathic skeletal hyperostosis and seronegative spondylarthropathies, is a T cell-independent process.

Secretion of Tumor-Specific Antigen by Myeloma Cells Is Required for Cancer Immunosurveillance by CD4+ T Cells

Tumor-specific CD4(+) T cells orchestrate the adaptive immune responses against cancer. We have previously shown that CD4(+) T cells recognize MHC class II-negative myeloma cells indirectly by collaborating with tumor-infiltrating macrophages. We, here, hypothesize that this critical step may be dependent on secretion of tumor-specific antigens by cancer cells. This was investigated using T-cell receptor-transgenic mice, in which CD4(+) T cells mediate rejection of syngeneic MOPC315 myeloma cells. We analyzed the immune response against myeloma cell variants, which either secrete or retain intracellularly a tumor-specific idiotypic (Id) antigen. Our results reveal that CD4(+) T cells helped by macrophages are capable of detecting nonsecreted tumor antigens from MHC class II-negative cancer cells. However, Id secretion was required for successful myeloma immunosurveillance. Antigen secretion resulted in stronger priming of naive myeloma-specific CD4(+) T cells in tumor-draining lymph nodes. Secretion of antigen by at least some cancer cells within a tumor was shown to facilitate immunosurveillance. Treatment by local injection of purified tumor-specific antigen successfully enhanced immunity against nonsecreting myeloma cells. Collectively, the data indicate that antigen concentration within the tumor extracellular matrix must reach a certain threshold to allow successful cancer immunosurveillance by CD4(+) T cells.

Role of gamma/delta T cell receptor-expressing lymphocytes in cutaneous infection caused by Staphylococcus aureus

The high number of γ/δ‐expressing T cells found in the epithelial lining layer suggests that they form a first line of defence against invading pathogens. To evaluate the role of γ/δ T cell‐receptor (TCR)‐expressing cells in cutaneous infection caused by Staphylococcus aureus, mice lacking γ/δ‐expressing T cells (TCRδ−/−) were inoculated intradermally with S. aureus, and compared with S. aureus‐infected congeneic TCRδ+/− control mice. The number of bacteria recovered from the skin of TCRδ−/− mice was significantly higher (P = 0·0071) at early time‐points after inoculation compared to the number of bacteria isolated from infected TCRδ+/− congeneic controls. Nevertheless, inflammatory responses measured as serum IL‐6 levels, were significantly lower in TCRδ−/− mice than in the control group. A possible explanation for this discrepancy was the observation of significantly decreased overall numbers of infiltrating cutaneous T lymphocytes, which are important producers of IL‐6. These results support the notion that the γ/δ‐expressing T cells that reside at the epithelial lining layer of the skin is of importance for early containment of the bacteria, thereby limiting their replication and spread.

Role of glycopeptide-specific T cells in collagen-induced arthritis: an example how post-translational modification of proteins may be involved in autoimmune disease

Immunization of mice with type II collagen (CII), a cartilage-restricted protein, leads to collagen-induced arthritis (CIA), a model for rheumatoid arthritis (RA). CIA symptoms consist of an erosive joint inflammation caused by an autoimmune attack, mediated by both T and B lymphocytes. CD4+ αβ T cells play a central role in CIA, both by helping B cells to produce anti-CII antibodies, and by interacting with other cells in the joints, eg macrophages. In H-2q mice, most CII-specific CD4+ T cells recognize the CII(256-270) peptide presented on the major histocompatibility complex (MHC) class II Aq molecule. Post-translational modifications (hydroxylation and variable glycosylation) of the lysine residue at position 264 of CII generate at least four different T-cell determinants that are specifically recognized by distinct T-cell subsets. Most T cells recognize CII(256-270) glycosylated with the monosaccharide galactose, which is consequently imrnuno-dominant in CIA. Recent studies indicate that the arthritogenic T cells in CIA are glycopeptide-specific, suggesting that induction of self-tolerance may be rendered more difficult by glycosylation of CII. These data open the possibility that autoimmune disease may be caused by the creation of new epitopes by post-translational modification of proteins under circumstances such as trauma, inflammation or ageing.