Craig T. Morita

Direct presentation of nonpeptide prenyl pyrophosphate antigens to human γδ T cells

Abstract Human Vγ2Vδ2 + T cells recognize mycobacterial non-peptide antigens, such as isopentenyl pyrophosphate, and their synthetic analogs, such as monoethyl phosphate, through a TCR-dependent process. Here, we examine the presentation of these antigens. Vγ2Vδ2 + T cells recognized secreted prenyl pyrophosphate antigens in the absence of other accessory cells but, under such conditions, required T cell-T cell contact. Recognition required neither the expression of classical MHC class I, MHC class II, or CD1a, CD1b, and CD1c molecules, nor MHC class I or class II peptide loading pathways. Fixed accessory cells also presented the prenyl pyrophosphate antigens to γδ T cells. Thus, in contrast with the presentation of conventional peptide antigens, protein antigens, and superantigens to αβ T cells, prenyl pyrophosphate antigens are presented to γδ T cells through a novel extracellular pathway that does not require antigen uptake, antigen processing, or MHC class I or class II expression. This pathway allows for the rapid recognition of bacteria by γδ T cells and suggests that γδ T cells play a role in the early response to bacterial infection.

MICA Engagement by Human Vγ2Vδ2 T Cells Enhances Their Antigen-Dependent Effector Function

Vgamma2Vdelta2 T cells comprise 2%-5% of human peripheral blood T cells, recognize ubiquitous nonpeptide antigens, and expand up to 50-fold during microbial infection. It is not clear why these Vgamma2Vdelta2 T cells expand only after microbial infection. We show here that the stress-inducible molecule, MICA, is induced on the surface of dendritic and epithelial cells by infection with M. tuberculosis in vitro and in vivo. MICA engagement by the activating receptor, NKG2D, present on Vgamma2Vdelta2 T cells, resulted in a substantial enhancement of the TCR-dependent Vgamma2Vdelta2 T cell response to nonpeptide antigens and protein superantigens alike. Thus, a MICA-NKG2D interaction may be necessary for an effective innate immune response to microbe-associated antigens that also are constitutively present in vivo.

Human gamma delta T cells recognize alkylamines derived from microbes, edible plants, and tea: implications for innate immunity.

Approximately 4% of peripheral blood T cells in humans express a T cell receptor with markedly restricted germline gene segment usage (V gamma 2 V delta 2). Remarkably, these T cells expand 2- to 10-fold (8%-60% of all circulating T cells) during many microbial infections. We show here that these T cells recognize a family of naturally occurring primary alkylamines in a TCR-dependent manner. These antigenic alkylamines are secreted to millimolar concentrations in bacterial supernatants and are found in certain edible plants. Given the large numbers of memory V gamma 2 V delta 2 T cells in adult humans, recognition of alkylamine antigens offers the immune system a response of the magnitude of major superantigens for alpha beta T cells and may bridge the gap between innate and adaptive immunity.

Nonpeptide antigens, presentation mechanisms, and immunological memory of human Vγ2Vδ2 T cells: discriminating friend from foe through the recognition of prenyl pyrophosphate antigens

Summary:  Human Vγ2Vδ2 T cells play important roles in mediating immunity against microbial pathogens and have potent anti‐tumor activity. Vγ2Vδ2 T cells recognize the pyrophosphorylated isoprenoid intermediates (E)‐4‐hydroxy‐3‐methyl‐but‐2‐enyl pyrophosphate (HMBPP), an intermediate in the foreign 2‐C‐methyl‐d‐erythritol 4‐phosphate (MEP) pathway, and isopentenyl pyrophosphate (IPP), an intermediate in the self‐mevalonate pathway. Infection with bacteria and protozoa using the MEP pathway leads to the rapid expansion of Vγ2Vδ2 T cells to very high numbers through preferential recognition of HMBPP. Activated Vγ2Vδ2 T cells produce proinflammatory cytokines and chemokines, kill infected cells, secrete growth factors for epithelial cells, and present antigens to αβ T cells. Vγ2Vδ2 T cells can also recognize high levels of IPP in certain tumors and in cells treated with pharmacological agents, such as bisphosphonates and alkylamines, that block farnesyl pyrophosphate synthase. Activated Vγ2Vδ2 T cells are able to kill most tumor cells because of recognition by T‐cell receptor and natural killer receptors. The ubiquitous nature of the antigens converts essentially all Vγ2Vδ2 T cells to memory cells at an early age. Thus, primary infections with HMBPP‐producing bacteria are perceived by Vγ2Vδ2 T cells as a repeat infection. Extensive efforts are underway to harness these cells to treat a variety of cancers and to provide microbial immunity.

The Syk family of protein tyrosine kinases in T-cell activation and development.

Summary: The processes of T‐cell development and activation employ similar immature and mature receptors as well as similar signal transduction pathways to achieve different outcomes. Many signaling molecules are shared between [he receptor signaling pathways, including two families of cytoplasmic protein tyrosine kinases, the Src family and the Syk family. The two Syk family members expressed in T cells, Syk and ZAP‐70, are structurally similar but are expressed at different times during rhymic development and during T‐cell activation. These two kinases, althongh they share many physical features, differ in terms of biochemical activity and regulation. We discuss the overlapping and distinct characteristics of Syk and ZAP‐70 in T‐cell signaling and the potential biological importance of their differences.

CD1-mediated γ/δ T Cell Maturation of Dendritic Cells

Immature myeloid dendritic cells (DCs) express only low levels of major histocompatibility complex (MHC) class II but express high levels of CD1 a, b, and c antigen-presenting molecules at the cell surface. As Vδ1+ γ/δ T cells are the main tissue subset of γ/δ T cells and they are known to recognize CD1c in the absence of specific foreign antigen recognition, we examined the possible interaction of these T cells with immature DCs. We show that CD1-restricted γ/δ T cells can mediate the maturation of DCs. DC maturation required cell–cell contact and could be blocked by antibodies against CD1c. The maturation process was partially mediated by tumor necrosis factor α. Importantly, immature DCs matured in the presence of lipopolysaccharide and CD1-restricted γ/δ T cells produced bioactive interleukin-12p70. In addition, these DCs were able to efficiently present peptide antigens to naive CD4+ T cells. CD1-restricted γ/δ T cell recognition of immature DCs provides the human immune system with the capacity to rapidly generate a pool of mature DCs early during microbial invasion. This may be an important source of critical host signals for T helper type 1 polarization of antigen-specific naive T cells and the subsequent adaptive immune response.

Antigen recognition by human γδ T cells: pattern recognition by the adaptive immune system

i Division of Rheumatology, Department of Internal Medicine, and the Interdisciplinary Group in Immunology, EMRB 340F, University of Iowa, Iowa City, IA 52242, USA 2 Center for Advanced Research in Biotechnology, University of Maryland Biotechnology Institute, 9600 Gudelsky Drive, Rockville, MD 20850, USA 3 Division of Rheumatology, Immunology, and Allergy, Department of Medicine, Brigham and Womens Hospital, Harvard Medical School, Smith 5th Floor, 75 Francis Street, Boston, MA 02115, USA

Cytokine Requirements for the Differentiation and Expansion of IL-17A– and IL-22–Producing Human Vγ2Vδ2 T Cells

Human γδ T cells expressing the Vγ2Vδ2 TCR play important roles in immune responses to microbial pathogens by monitoring prenyl pyrophosphate isoprenoid metabolites. Most adult Vγ2Vδ2 cells are memory cytotoxic cells that produce IFN-γ. Recently, murine γδ T cells were found to be major sources of IL-17A in antimicrobial and autoimmune responses. To determine if primate γδ T cells play similar roles, we characterized IL-17A and IL-22 production by Vγ2Vδ2 cells. IL-17A–producing memory Vγ2Vδ2 cells exist at low but significant frequencies in adult humans (1:2762 T cells) and at even higher frequencies in adult rhesus macaques. Higher levels of Vγ2Vδ2 cells produce IL-22 (1:1864 T cells), although few produce both IL-17A and IL-22. Unlike adult humans, in whom many IL-17A+ Vγ2Vδ2 cells also produce IFN-γ (Tγδ1/17), the majority of adult macaques IL-17A+ Vδ2 cells (Tγδ17) do not produce IFN-γ. To define the cytokine requirements for Tγδ17 cells, we stimulated human neonatal Vγ2Vδ2 cells with the bacterial Ag, (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate, and various cytokines and mAbs in vitro. We find that IL-6, IL-1β, and TGF-β are required to generate Tγδ17 cells in neonates, whereas Tγδ1/17 cells additionally required IL-23. In adults, memory Tγδ1/17 and Tγδ17 cells required IL-23, IL-1β, and TGF-β, but not IL-6. IL-22–producing cells showed similar requirements. Both neonatal and adult IL-17A+ Vγ2Vδ2 cells expressed elevated levels of retinoid-related orphan receptor γt. Our data suggest that, like Th17 αβ T cells, Vγ2Vδ2 T cells can be polarized into Tγδ17 and Tγδ1/17 populations with distinct cytokine requirements for their initial polarization and later maintenance.

Butyrophilin 3A1 Plays an Essential Role in Prenyl Pyrophosphate Stimulation of Human Vγ2Vδ2 T Cells

Most human γδ T cells express Vγ2Vδ2 TCRs and play important roles in microbial and tumor immunity. Vγ2Vδ2 T cells are stimulated by self- and foreign prenyl pyrophosphate intermediates in isoprenoid synthesis. However, little is known about the molecular basis for this stimulation. We find that a mAb specific for butyrophilin 3 (BTN3)/CD277 Ig superfamily proteins mimics prenyl pyrophosphates. The 20.1 mAb stimulated Vγ2Vδ2 T cell clones regardless of their functional phenotype or developmental origin and selectively expanded blood Vγ2Vδ2 T cells. The γδ TCR mediates 20.1 mAb stimulation because IL-2 is released by β− Jurkat cells transfected with Vγ2Vδ2 TCRs. 20.1 stimulation was not due to isopentenyl pyrophosphate (IPP) accumulation because 20.1 treatment of APC did not increase IPP levels. In addition, stimulation was not inhibited by statin treatment, which blocks IPP production. Importantly, small interfering RNA knockdown of BTN3A1 abolished stimulation by IPP that could be restored by re-expression of BTN3A1 but not by BTN3A2 or BTN3A3. Rhesus monkey and baboon APC presented HMBPP and 20.1 to human Vγ2Vδ2 T cells despite amino acid differences in BTN3A1 that localize to its outer surface. This suggests that the conserved inner and/or top surfaces of BTN3A1 interact with its counterreceptor. Although no binding site exists on the BTN3A1 extracellular domains, a model of the intracellular B30.2 domain predicts a basic pocket on its binding surface. However, BTN3A1 did not preferentially bind a photoaffinity prenyl pyrophosphate. Thus, BTN3A1 is required for stimulation by prenyl pyrophosphates but does not bind the intermediates with high affinity.

T-cell recognition of non-peptide antigens

Studies of two distinct human T-cell systems have provided the exciting finding that T cells are able to recognize non-peptide antigens: gammadelta T cells have been shown to recognize isopentenyl pyrophosphate and related structures and human CD1 has been shown to present microbial lipids and lipoglycans such as mycolic acids and lipoarabinomannan to T cells. T-cell responses to these non-peptide antigens should provide a strategic target for immunologic intervention in infectious disease.