Qa-1 restricted recognition of foreign antigen by a gamma delta T-cell hybridoma

Aging unconventionally: γδ T cells, iNKT cells, and MAIT cells in aging

Unconventional T cells include γδ T cells, invariant Natural Killer T cells (iNKT) cells and Mucosal Associated Invariant T (MAIT) cells, which are distinguished from conventional T cells by their recognition of non-peptide ligands presented by non-polymorphic antigen presenting molecules and rapid effector functions that are pre-programmed during their development. Here we review current knowledge of the effect of age on unconventional T cells, from early life to old age, in both mice and humans. We then discuss the role of unconventional T cells in age-associated diseases and infections, highlighting the similarities between members of the unconventional T cell family in the context of aging.

Diversity in recognition and function of human γδ T cells

As interest increases in harnessing the potential power of tissue-resident cells for human health and disease, γδ T cells have been thrust into the limelight due to their prevalence in peripheral tissues, their sentinel-like phenotypes, and their unique antigen recognition capabilities. This review focuses primarily on human γδ T cells, highlighting their distinctive characteristics including antigen recognition, function, and development, with an emphasis on where they differ from their αβ T cell comparators, as well as from γδ T cell populations in the mouse. We review the antigens that have been identified thus far to regulate members of the human Vδ1 population and discuss what players are involved in transducing phosphoantigen-mediated signals to human Vγ9Vδ2 T cells. We also briefly review distinguishing features of these cells in terms of TCR signaling, use of coreceptor and costimulatory molecules and their development. These cells have great potential to be harnessed in a clinical setting, but caution must be taken to understand their unique capabilities and how they differ from the populations to which they are commonly compared.

Ligand recognition by the γδ TCR and discrimination between homeostasis and stress conditions

T lymphocytes comprise cells expressing either an αβ or a γδ TCR. The riddle how αβ TCRs are triggered by specific peptides presented in the context of MHC was elucidated some time ago. In contrast, the mechanisms that underlie antigen recognition by γδ TCRs are still baffling the scientific community. It is clear that activation of γδ TCRs does not necessarily depend on MHC antigen presentation. To date, diverse and largely host-cell-derived molecules have been identified as cognate antigens for the γδ TCR. However, for most γδ TCRs, the activating ligand is still unknown and many open questions with regard to physiological relevance and generalizable concepts remain. Especially the question of how γδ T cells can distinguish homeostatic from stress conditions via their TCR remains largely unresolved. Recent discoveries in the field might have paved the way towards a better understanding of antigen recognition by the γδ TCR and have made it conceivable to revise the current knowledge and contextualize the new findings.

The Aging of γδ T Cells

In the coming decades, many developed countries in the world are expecting the “greying” of their populations. This phenomenon poses unprecedented challenges to healthcare systems. Aging is one of the most important risk factors for infections and a myriad of diseases such as cancer, cardiovascular and neurodegenerative diseases. A common denominator that is implicated in these diseases is the immune system. The immune system consists of the innate and adaptive arms that complement each other to provide the host with a holistic defense system. While the diverse interactions between multiple arms of the immune system are necessary for its function, this complexity is amplified in the aging immune system as each immune cell type is affected differently—resulting in a conundrum that is especially difficult to target. Furthermore, certain cell types, such as γδ T cells, do not fit categorically into the arms of innate or adaptive immunity. In this review, we will first introduce the human γδ T cell family and its ligands before discussing parallels in mice. By covering the ontogeny and homeostasis of γδ T cells during their lifespan, we will better capture their evolution and responses to age-related stressors. Finally, we will identify knowledge gaps within these topics that can advance our understanding of the relationship between γδ T cells and aging, as well as age-related diseases such as cancer.

γδ T Cell Update: Adaptate Orchestrators of Immune Surveillance

As interest in γδ T cells grows rapidly, what key points are emerging, and where is caution warranted? γδ T cells fulfill critical functions, as reflected in associations with vaccine responsiveness and cancer survival in humans and ever more phenotypes of γδ T cell-deficient mice, including basic physiological deficiencies. Such phenotypes reflect activities of distinct γδ T cell subsets, whose origins offer interesting insights into lymphocyte development but whose variable evolutionary conservation can obfuscate translation of knowledge from mice to humans. By contrast, an emerging and conserved feature of γδ T cells is their "adaptate" biology: an integration of adaptive clonally-restricted specificities, innate tissue-sensing, and unconventional recall responses that collectively strengthen host resistance to myriad challenges. Central to adaptate biology are butyrophilins and other γδ cell regulators, the study of which should greatly enhance our understanding of tissue immunogenicity and immunosurveillance and guide intensifying clinical interest in γδ cells and other unconventional lymphocytes.

The complex existence of γδ T cells following transplantation: the good, the bad and the simply confusing

Gamma delta (γδ) T cells are a highly heterogeneous population of lymphocytes that exhibit innate and adaptive immune properties. Despite comprising the majority of residing lymphocytes in many organs, the role of γδ T cells in transplantation outcomes is under‐researched. γδ T cells can recognise a diverse array of ligands and exert disparate effector functions. As such, they may potentially contribute to both allograft acceptance and rejection, as well as impacting on infection and post‐transplant malignancy. Here, we review the current literature on the role and function of γδ T cells following solid organ and hematopoietic stem cell transplantation.

Down Regulation of Qa Gene Expression on Drug-Modified Tumor Cells

Background

Mouse leukemia, L1210, strongly enhances its immunogenicity following in vivo treatment with 5-(3-3′-dimethyl-1-triazeno) imidazole-4-carboxamide (DTIC). Previous experiments have shown that transformed cells elicit a cell-mediated response accountable for rejection and resistance to a subsequent injection of parental tumor into a syngeneic host. L1210 expresses classical H-2 class I molecules, and since it has been shown that DTIC treatment does not modify the expression of these molecules, this is a suitable model to study nonclassical class I antigens, such as Qa2 glycoproteins, and their potential role in tumoregenicity.

Methods

Cloned cells from L1210 were treated with DTIC and then H-2D, and Qa antigen expression was studied on four clones, before and after xenogenization with DTIC.

Results and conclusions

a strong decrease of Qa2 molecule expression was demonstrated by radioimmunoassay and immunofluorescent staining and was confirmed by FACS and 2D-gel analysis. The presence or the absence of Qa antigens on tumor cells could thus be involved in tolerance or rejection of tumor cells in syngeneic animals.

γδ T cell responses: How many ligands will it take till we know?

γδ T cells constitute a sizeable and non-redundant fraction of the total T cell pool in all jawed vertebrates, but in contrast to conventional αβ T cells they are not restricted by classical MHC molecules. Progress in our understanding of the role of γδ T cells in the immune system has been hampered, and is being hampered, by the considerable lack of knowledge regarding the antigens γδ T cells respond to. The past few years have seen a wealth of data regarding the TCR repertoires of distinct γδ T cell populations and a growing list of confirmed and proposed molecules that are recognised by γδ T cells in different species. Yet, the physiological contexts underlying the often restricted TCR usage and the chemical diversity of γδ T cell ligands remain largely unclear, and only few structural studies have confirmed direct ligand recognition by the TCR. We here review the latest progress in the identification and validation of putative γδ T cell ligands and discuss the implications of such findings for γδ T cell responses in health and disease.

The co-stimulatory effects of MyD88-dependent Toll-like receptor signaling on activation of murine γδ T cells

γδ T cells express several different toll-like receptor (TLR)s. The role of MyD88- dependent TLR signaling in TCR activation of murine γδ T cells is incompletely defined. Here, we report that Pam3CSK4 (PAM, TLR2 agonist) and CL097 (TLR7 agonist), but not lipopolysaccharide (TLR4 agonist), increased CD69 expression and Th1-type cytokine production upon anti-CD3 stimulation of γδ T cells from young adult mice (6-to 10-week-old). However, these agonists alone did not induce γδ T cell activation. Additionally, we noted that neither PAM nor CL097 synergized with anti-CD3 in inducing CD69 expression on γδ T cells of aged mice (21-to 22-month-old). Compared to young γδ T cells, PAM and CL097 increased Th-1 type cytokine production with a lower magnitude from anti-CD3- stimulated, aged γδ T cells. Vγ1⁺ and Vγ4⁺ cells are two subpopulations of splenic γδ T cells. PAM had similar effects in anti-CD3-activated control and Vγ4⁺ subset- depleted γδ T cells; whereas CL097 induced more IFN-γ production from Vγ4⁺ subset-depleted γδ T cells than from the control group. Finally, we studied the role of MyD88-dependent TLRs in γδ T cell activation during West Nile virus (WNV) infection. γδ T cell, in particular, Vγ1⁺ subset expansion was significantly reduced in both MyD88- and TLR7- deficient mice. Treatment with TLR7 agonist induced more Vγ1⁺ cell expansion in wild-type mice during WNV infection. In summary, these results suggest that MyD88-dependent TLRs provide co-stimulatory signals during TCR activation of γδ T cells and these have differential effects on distinct subsets.

γδ T cells recognize the insulin B:9-23 peptide antigen when it is dimerized through thiol oxidation

The insulin peptide B:9-23 is a natural antigen in the non-obese diabetic (NOD) mouse model of type 1 diabetes (T1D). In addition to αβ T cells and B cells, γδ T cells recognize the peptide and infiltrate the pancreatic islets where the peptide is produced within β cells. The peptide contains a cysteine in position 19 (Cys19), which is required for the γδ but not the αβ T cell response, and a tyrosine in position 16 (Tyr16), which is required for both. A peptide-specific mAb, tested along with the T cells, required neither of the two amino acids to bind the B:9-23 peptide. We found that γδ T cells require Cys19 because they recognize the peptide antigen in an oxidized state, in which the Cys19 thiols of two peptide molecules form a disulfide bond, creating a soluble homo-dimer. In contrast, αβ T cells recognize the peptide antigen as a reduced monomer, in complex with the MHCII molecule I-A(g7). Unlike the unstructured monomeric B:9-23 peptide, the γδ-stimulatory homo-dimer adopts a distinct secondary structure in solution, which differs from the secondary structure of the corresponding portion of the native insulin molecule. Tyr16 is required for this adopted structure of the dimerized insulin peptide as well as for the γδ response to it. This observation is consistent with the notion that γδ T cell recognition depends on the secondary structure of the dimerized insulin B:9-23 antigen.

γδ T cells: first line of defense and beyond

γδ T cells, αβ T cells, and B cells are present together in all but the most primitive vertebrates, suggesting that each population contributes to host immune competence uniquely and that all three are necessary for maintaining immune competence. Functional and molecular analyses indicate that in infections, γδ T cells respond earlier than αβ T cells do and that they emerge late after pathogen numbers start to decline. Thus, these cells may be involved in both establishing and regulating the inflammatory response. Moreover, γδ T cells and αβ T cells are clearly distinct in their antigen recognition and activation requirements as well as in the development of their antigen-specific repertoire and effector function. These aspects allow γδ T cells to occupy unique temporal and functional niches in host immune defense. We review these and other advances in γδ T cell biology in the context of their being the major initial IL-17 producers in acute infection.

Diversity of γδ T-cell antigens

In the last two decades, it has become clear that γδ T cells recognize a diverse array of antigens including self and foreign, large and small, and peptidic and non-peptidic molecules. In this respect, γδ antigens as a whole resemble more the antigens recognized by antibodies than those recognized by αβ T cells. Because of this antigenic diversity, no single mechanism-such as the major histocompatibility complex (MHC) restriction of αβ T cells-is likely to provide a basis for all observed T-cell antigen receptor (TCR)-dependent γδ T-cell responses. Furthermore, available evidence suggests that many individual γδ T cells are poly-specific, probably using different modes of ligand recognition in their responses to unrelated antigens. While posing a unique challenge in the maintenance of self-tolerance, this broad reactivity pattern might enable multiple overlapping uses of γδ T-cell populations, and thus generate a more efficient immune response.

Regulatory T-cells: diverse phenotypes integral to immune homeostasis and suppression

Regulatory T-cells (T(REG)) are diverse populations of lymphocytes that regulate the adaptive immune response in higher vertebrates. T(REG) delete autoreactive T-cells, induce tolerance, and dampen inflammation. T(REG) cell deficiency in humans (i.e., IPEX [Immunodysregulation, Polyendocrinopathy and Enteropathy, X-linked syndrome]) and animal models (e.g., "Scurfy" mouse) is associated with multisystemic autoimmune disease. T(REG) in humans and laboratory animal species are similar in type and regulatory function. A molecular marker of and the cell lineage specification factor for T(REG) is FOXP3, a forkhead box transcription factor. CD4(+) T(REG) are either natural (nT(REG)), which are thymus-derived CD4(+)CD25(+)FOXP3(+) T-cells, or inducible (i.e., Tr1 cells that secrete IL-10, Th3 cells that secrete TGF-β and IL-10, and Foxp3(+) Treg). The proinflammatory Th17 subset has been a major focus of research. T(H)17 CD4(+) effector T-cells secrete IL-17, IL-21, and IL-22 in autoimmune and inflammatory disease, and are dynamically balanced with T(REG) cell development. Other lymphocyte subsets with regulatory function include: inducible CD8(+) T(REG), CD3(+)CD4(-)CD8(-) T(REG) (double-negative), CD4(+)Vα14(+) (NKT(REG)), and γδ T-cells. T(REG) have four regulatory modes of action: secretion of inhibitory cytokines (e.g., IL-10 and TGF-β), granzyme-perforin-induced apoptosis of effector lymphocytes, depriving effector T-cells of cytokines leading to apoptosis, or inhibition of dendritic cell function. The role of T(REG) in mucosal sites, inflammation/infection, pregnancy, and cancer as well as a review of T(REG) as a modulatory target in drug development will be covered.

A structural basis for antigen presentation by the MHC class Ib molecule, Qa-1b

The primary function of the monomorphic MHC class Ib molecule Qa-1(b) is to present peptides derived from the leader sequences of other MHC class I molecules for recognition by the CD94-NKG2 receptors expressed by NK and T cells. Whereas the mode of peptide presentation by its ortholog HLA-E, and subsequent recognition by CD94-NKG2A, is known, the molecular basis of Qa-1(b) function is unclear. We have assessed the interaction between Qa-1(b) and CD94-NKG2A and shown that they interact with an affinity of 17 μM. Furthermore, we have determined the structure of Qa-1(b) bound to the leader sequence peptide, Qdm (AMAPRTLLL), to a resolution of 1.9 Å and compared it with that of HLA-E. The crystal structure provided a basis for understanding the restricted peptide repertoire of Qa-1(b). Whereas the Qa-1(b-AMAPRTLLL) complex was similar to that of HLA-E, significant sequence and structural differences were observed between the respective Ag-binding clefts. However, the conformation of the Qdm peptide bound by Qa-1(b) was very similar to that of peptide bound to HLA-E. Although a number of conserved innate receptors can recognize heterologous ligands from other species, the structural differences between Qa-1(b) and HLA-E manifested in CD94-NKG2A ligand recognition being species specific despite similarities in peptide sequence and conformation. Collectively, our data illustrate the structural homology between Qa-1(b) and HLA-E and provide a structural basis for understanding peptide repertoire selection and the specificity of the interaction of Qa-1(b) with CD94-NKG2 receptors.

Peptide antigens for gamma/delta T cells

γδ T cells express adaptive antigen receptors encoded by rearranging genes. Their diversity is highest in the small region of TCR V-J junctions, especially in the δ chain, which should enable the γδ TCRs to distinguish differences in small epitopes. Indeed, recognition of small molecules, and of an epitope on a larger protein has been reported. Responses to small non-peptides known as phospho-antigens are multi-clonal yet limited to a single γδ T cell subset in humans and non-human primates. Responses to small peptides are multi-clonal or oligo-clonal, include more than one subset of γδ T cells, and occur in rodents and primates. However, less effort has been devoted to investigate the peptide responses. To settle the questions of whether peptides can be ligands for the γδ TCRs, and whether responses to small peptides might occur normally, peptide binding will have to be demonstrated, and natural peptide ligands identified.

αβ versus γδ fate choice: counting the T-cell lineages at the branch point

Both αβ and γδ T cells develop in the thymus from a common progenitor. Historically distinguished by their T-cell receptor (TCR), these lineages are now defined on the basis of distinct molecular programs. Intriguingly, in many transgenic and knockout systems these programs are mismatched with the TCR type, leading to the development of γδ lineage cells driven by αβTCR and vice versa. These puzzling observations were recently explained by the demonstration that TCR signal strength, rather than TCR type per se, instructs lineage fate, with stronger TCR signal favoring γδ and weaker signal favoring αβ lineage fates. These studies also highlighted the ERK (extracellular signal regulated kinase)-Egr (early growth response)-Id3 (inhibitor of differentiation 3) axis as a potential molecular switch downstream of TCR that determines lineage choice. Indeed, removal of Id3 was sufficient to redirect TCRγδ transgenic cells to the αβ lineage, even in the presence of strong TCR signal. However, in TCR non-transgenic Id3 knockout mice the overall number of γδ lineage cells was increased due to an outgrowth of a Vγ1Vδ6.3 subset, suggesting that not all γδ T cells depend on this molecular switch for lineage commitment. Thus, the γδ lineage may in fact be a collection of two or more lineages not sharing a common molecular program and thus equipollent to the αβ lineage. TCR signaling is not the only factor that is required for development of αβ and γδ lineage cells; other pathways, such as signaling from Notch and CXCR4 receptors, cooperate with the TCR in this process.

γδ T cell receptor ligands and modes of antigen recognition

T lymphocytes expressing the γδ-type of T cell receptors (TCRs) for antigens contribute to all aspects of immune responses, including defenses against viruses, bacteria, parasites and tumors, allergy and autoimmunity. Multiple subsets have been individualized in humans as well as in mice and they appear to recognize in a TCR-dependent manner antigens as diverse as small non-peptidic molecules, soluble or membrane-anchored polypeptides and molecules related to MHC antigens on cell surfaces, implying diverse modes of antigen recognition. We review here the γδ TCR ligands which have been identified along the years and their characteristics, with emphasis on a few systems which have been extensively studied such as human γδ T cells responding to phosphoantigens or murine γδ T cells activated by allogeneic MHC antigens. We discuss a speculative model of antigen recognition involving simultaneous TCR recognition of MHC-like and non-MHC ligands which could fit with most available data and shares many similarities with the classical model of MHC-restricted antigen recognition for peptides or lipids by T cells subsets with αβ-type TCRs.

gammadeltaTCR ligands and lineage commitment

Two major T lymphocyte lineages--alphabeta and gammadelta T cells--develop in the thymus from common precursors. Differentiation of both lineages requires signals coming from TCRs. Development of alphabeta T cells is driven at early stages by signaling from the pre-TCR, most likely in a ligand-independent fashion, and later--by signals delivered by alphabetaTCRs binding to their ligands--classical or non-classical MHC molecules. gammadelta lineage cells likewise require TCR signaling for their differentiation. Recent work from several groups suggests that TCR signaling not only ensures the developmental progression towards alphabeta and gammadelta lineages but that signal strength instructs lineage fate: weaker TCR signal results in alphabeta and stronger--in gammadelta lineage commitment. However, as most gammadeltaTCRs remain orphan receptors, it is still debated whether strong signals from gammadeltaTCRs in development are generated in a ligand-dependent manner (as in the case of alphabetaTCRs), ligand-independent manner (as for pre-TCR) or both. Here we summarize evidence supporting a possible role for ligands in gammadelta T cell lineage commitment and the generation of gammadelta sublineages.

Antigen-restricted gammadelta T-cell receptors?

After more than two decades of investigation, the biological role of the gammadelta T-cell receptors (TCRs) remains elusive. In fact, a theory of ligand recognition is still lacking that accounts for their adaptable structure, their peripheral selection, and the observed responses of gammadelta T cells, which do not require immunization but only include cells sharing germline-encoded components of the TCR. Assuming that all gammadelta T cells recognize ligands by a common mechanism, we now propose that germline-encoded components of the gammadelta TCRs provide for the specific recognition of a select set of antigenic determinants (Ags) which appear on the cell surface in various molecular associations. Furthermore, we hypothesize that the adaptivity of the gammadelta TCRs serves to increase affinity for the molecules with which these Ags associate rather than for the Ags themselves. Here we outline this hypothetical mechanism and discuss its possible implications for thymic selection and potential for complementing known innate and adaptive mechanisms of immune defense.

Gammadelta T cells do not require fully functional cytotoxic pathways or the ability to recognize recipient alloantigens to prevent graft rejection

Gammadelta T cells are a unique and minor T-cell subset that differs from conventional alphabeta T cells by virtue of their tissue localization and antigen processing requirements. We have previously shown that ex vivo-activated gammadelta T cells are able to prevent graft rejection without causing clinically significant graft-versus-host disease (GVHD). In the present study, we examined how gammadelta T cells facilitate alloengraftment and to what extent mechanisms used by conventional alphabeta T cells are also used by gammadelta T cells. We observed that, unlike alphabeta T cells, for which CD8(+) T cells are primarily responsible for facilitating engraftment, purified CD8(+)gammadelta(+) T cells administered at the same fractional dose as for the unseparated activated gammadelta T-cell population were insufficient to prevent graft rejection. Furthermore, the ability to prevent graft rejection was not affected by the absence of fully functional fas ligand or perforin cytotoxic pathways, nor was it contingent on the ability of gammadelta T cells to recognize recipient major histocompatibility process alloantigens. Repetitive infusions of a suboptimal dose of gammadelta T cells however were able to rescue mice from graft rejection, suggesting that the persistence of these cells in vivo was critical in facilitating alloengraftment. These studies demonstrate that gammadelta T cells do not use mechanisms used by conventional nontolerant alphabeta T cells to prevent graft rejection. The ability of these cells to promote engraftment without causing GVHD further distinguishes these cells from alphabeta T cells and may be an attribute that can be exploited in the clinical transplantation setting.

γδ T cells — innate immune lymphocytes?

It is unclear what the antigen recognition determinants of gammadelta T-cell receptors (TCRs) are. Compared with immunoglobulin and alphabeta TCRs, gammadelta TCRs have the highest potential CDR3 diversity generated by VDJ recombination. However, gammadelta T-cell reactivities seem to segregate with V gene usage, which has been taken to suggest that rearrangement has little role in generating different antigen specificities. During the past year, the CDR3 regions were found to determine the antigen specificities of T10- and T22-reactive gammadelta TCRs, a surface protein complex was identified as a ligand for human phosphoantigen-reactive gammadelta T cells, and the first co-crystal structure of a gammadelta TCR bound to its ligand was reported. These advances warrant a fresh look at gammadelta T-cell antigen recognition.

MHC class Ib molecules bridge innate and acquired immunity

Our understanding of the classical MHC class I molecules (MHC class Ia molecules) has long focused on their extreme polymorphism. These molecules present peptides to T cells and are central to discrimination between self and non-self. By contrast, the functions of the non-polymorphic MHC class I molecules (MHC class Ib molecules) have been elusive, but emerging evidence reveals that, in addition to antigen presentation, MHC class Ib molecules are involved in immunoregulation. As we discuss here, the subset of MHC class Ib molecules that presents peptides to T cells bridges innate and acquired immunity, and this provides insights into the origins of acquired immunity.

Infection-induced expansion of a MHC Class Ib-dependent intestinal intraepithelial gammadelta T cell subset

Salmonella species invade the host via the intestinal epithelium. Hence, intestinal intraepithelial lymphocytes (iIELs) are potentially the first element of the immune system to encounter Salmonella during infection. In this study, we demonstrate, in a mouse model, the expansion of a CD8alphabeta(+)CD94(-)TCRgammadelta(+) T cell subset within the iIEL population in response to oral infection with virulent or avirulent Salmonella. This population can be detected 3 days following infection, represents up to 15% of the TCRgammadelta(+) iIELs, and is dependent on the MHC class Ib molecule T23 (Qa-1). Qa-1 is expressed by intestinal epithelial cells and thus accessible for iIEL recognition. Such cells may play a role in the early immune response to Salmonella.

Viral degradation of the MHC class I peptide loading complex

The murine gamma-herpesvirus-68 MK3 protein inhibits CD8(+) T cell recognition by ubiquitinating the cytoplasmic tails of classical MHC class I heavy chains. Here we show that MK3 also provides the first example of a protein that degrades tapasin and TAP. The degradation was MK3 RING finger dependent and primarily affected TAP. MK3 associated with TAP1 in the absence of tapasin or TAP2, suggesting that TAP1 was a primary binding partner in the peptide loading complex. TAP2 also played a major role in MK3 stability and function. By degrading TAP, therefore, MK3 limited its own expression. However, TAP degradation also broadened the MK3 inhibitory repertoire and achieved a remarkable resistance to MHC class I upregulation by interferon-gamma, suggesting that it represents a specific adaptation to immune evasion in lymphoid tissue.

Qa-1, a nonclassical class I histocompatibility molecule with roles in innate and adaptive immunity

Qa-1, a nonclassical class I histocompatibility molecule expressed in mice, predominantly assembles with a single nonameric peptide, Qdm, derived from the signal sequence of certain class Ia molecules. The Qa-1/Qdm complex is the primary ligand for CD94/NKG2A inhibitory receptors expressed on a major fraction of natural killer (NK) cells. Cells become susceptible to killing by NK cells under conditions where surface expression of the Qa-1/Qdm inhibitory ligand is reduced. The CD94/NKG2 "missing-self" recognition system serves as mechanism for removing cells that have abnormalities in the intracellular machinery required for assembly and expression of class I-peptides complexes, as a consequence of viral infection, for example. Despite its highly focused peptide-binding specificity, Qa-1 also has a capacity to act as an antigen-presentation molecule for CD8+ T cells. It appears that a small subpopulation of these T cells undergoes positive selection by interaction with Qa-1 in the thymus, and they maintain their specificity for Qa-1 after maturation. The role of these unusual T cells in adaptive immune responses remains to be defined.

Hybridomas expressing gammadelta T-cell receptors respond to cardiolipin and beta2-glycoprotein 1 (apolipoprotein H)

Hybridomas expressing murine gammadelta T-cell receptors were found to produce cytokines in response to cardiolipin (CL) and structurally related anionic phospholipids. This response required serum at concentrations related to the amount of CL in cultures. The purified serum factor, beta2-glycoprotein 1 (beta2-GP1) (apolipoprotein H), supported the CL response alone, whereas several other serum proteins and ovalbumin did not. beta2-GP1 is known to form complexes with anionic phospholipids, particularly CL, which are often recognized by pathological autoantibodies. We speculate that gammadelta T cells also recognize such complexes and that the hybridoma response reported here reflects this specificity.

A peptide from heat shock protein 60 is the dominant peptide bound to Qa-1 in the absence of the MHC class Ia leader sequence peptide Qdm

The MHC class Ib molecule Qa-1 binds specifically and predominantly to a single 9-aa peptide (AMAPRTLLL) derived from the leader sequence of many MHC class Ia proteins. This peptide is referred to as Qdm. In this study, we report the isolation and sequencing of a heat shock protein 60-derived peptide (GMKFDRGYI) from Qa-1. This peptide is the dominant peptide bound to Qa-1 in the absence of Qdm. A Qa-1-restricted CTL clone recognizes this heat shock protein 60 peptide, further verifying that it binds to Qa-1 and a peptide from the homologous Salmonella typhimurium protein GroEL (GMQFDRGYL). These observations have implications for how Qa-1 can influence NK cell and T cell effector function via the TCR and CD94/NKG2 family members, and how this effect can change under conditions that cause the peptides bound to Qa-1 to change.

MHC class I-dependent Vgamma4+ pulmonary T cells regulate alpha beta T cell-independent airway responsiveness

Mice exposed to aerosolized ovalbumin (OVA) develop increased airway responsiveness when deficient in gammadelta T cells. This finding suggests that gammadelta T cells function as negative regulators. The regulatory influence of gammadelta T cells is evident after OVA-sensitization and -challenge, and after OVA-challenge alone, but not in untreated mice. With aerosolized Abs to target pulmonary T cells, we now demonstrate that negative regulation of airway responsiveness is mediated by a small subpopulation of pulmonary gammadelta T cells. These cells express Vgamma4 and depend in their function on the presence of IFN-gamma and MHC class I. Moreover, their effect can be demonstrated in the absence of alphabeta T cells. This novel type of negative regulation seems to precede the development of the adaptive, antigen-specific allergic response.

Activation of murine epidermal V gamma 5/V delta 1-TCR(+) T cell lines by Glu-Tyr polypeptides

The physiologic role of gamma delta-T-cell-receptor-bearing cells and the T cell receptor ligands that they recognize is still poorly understood. Previous studies have suggested that one possible antigen for gamma delta-TCR(+) cells is the random copolymer poly-glutamic acid-tyrosine (poly-Glu-Tyr), because poly-Glu-Tyr-reactive gamma delta-TCR(+) hybridoma cells were produced from poly-Glu-Tyr-immunized mice. We have found, however, that clonal V gamma 5/V delta 1-TCR(+) epidermal T cell lines from nonimmune mice also respond to poly-Glu-Tyr by producing cytokines. Other amino acid homopolymers, copolymers, and tripolymers were not stimulatory for the V gamma 5/V delta 1-TCR(+) epidermal T cells, except for poly-glutamic acid-alanine-tyrosine (poly-Glu-Ala-Tyr). Of the poly-Glu-Tyr and poly-Glu-Ala-Tyr polymers, only those that contained Glu and Tyr in an equimolar ratio were stimulatory. The cytokine interleukin-2 was strictly required for the responses to poly-Glu-Ala-Tyr, whereas the responses to poly-Glu-Tyr were merely enhanced with interleukin-2. The response to poly-Glu-Tyr was also enhanced by crosslinking CD28 molecules with plate-bound anti-CD28 crosslinking antibody. This finding suggests that the poly-Glu-Tyr response has a partial dependence on CD28-mediated costimulation, a characteristic of TCR-dependent responses. Consistent with this observation, V gamma 5/V delta 1-TCR-loss variants of the epidermal T cell line could no longer respond to poly-Glu-Tyr. The unpredicted responses of epidermal gamma delta-TCR(+) T cells to poly-Glu-Tyr and poly-Glu-Ala-Tyr demonstrate that the functions of these cells potentially can be triggered by peptidic ligands, probably through a TCR-mediated process.

Analysis of Qa-1(b) peptide binding specificity and the capacity of CD94/NKG2A to discriminate between Qa-1-peptide complexes

The major histocompatibility complex class Ib protein, Qa-1(b), serves as a ligand for murine CD94/NKG2A natural killer (NK) cell inhibitory receptors. The Qa-1(b) peptide-binding site is predominantly occupied by a single nonameric peptide, Qa-1 determinant modifier (Qdm), derived from the leader sequence of H-2D and L molecules. Five anchor residues were identified in this study by measuring the peptide-binding affinities of substituted Qdm peptides in experiments with purified recombinant Qa-1(b). A candidate peptide-binding motif was determined by sequence analysis of peptides eluted from Qa-1 that had been folded in the presence of random peptide libraries or pools of Qdm derivatives randomized at specific anchor positions. The results indicate that Qa-1(b) can bind a diverse repertoire of peptides but that Qdm has an optimal primary structure for binding Qa-1(b). Flow cytometry experiments with Qa-1(b) tetramers and NK target cell lysis assays demonstrated that CD94/NKG2A discriminates between Qa-1(b) complexes containing peptides with substitutions at nonanchor positions P4, P5, or P8. Our findings suggest that it may be difficult for viruses to generate decoy peptides that mimic Qdm and raise the possibility that competitive replacement of Qdm with other peptides may provide a novel mechanism for activation of NK cells.

Host immune response to intracellular bacteria: A role for MHC-linked class-Ib antigen-presenting molecules

MHC-linked class-Ib molecules are a subfamily of class-I molecules that display limited genetic polymorphism. At one time these molecules were considered to have an enigmatic function. However, recent studies have shown that MHC-linked class-Ib molecules can function as antigen presentation structures that bind bacteria-derived epitopes for recognition by CD8+ effector T cells. This role for class-Ib molecules has been demonstrated across broad classes of intracellular bacteria including Listeria moncytogenes, Salmonella typhimurium, and Mycobacterium tuberculosis. Additionally, evidence is emerging that MHC-linked class-Ib molecules also serve an integral role as recognition elements for NK cells as well as several TCR alpha/beta and TCR gamma/delta T-cell subsets. Thus, MHC-linked class-Ib molecules contribute to the host immune response by serving as antigen presentation molecules and recognition ligands in both the innate and adaptive immune response to infection. In this review, we will attempt to summarize the work that supports a role for MHC-linked class-Ib molecules in the host response to infection with intracellular bacteria.

Response of murine gamma delta T cells to the synthetic polypeptide poly-Glu50Tyr50

Random heterocopolymers of glutamic acid and tyrosine (pEY) evoke strong, genetically controlled immune responses in certain mouse strains. We found that pE50Y50 also stimulated polyclonal proliferation of normal gamma delta, but not alpha beta, T cells. Proliferation of gamma delta T cells did not require prior immunization with this Ag nor the presence of alpha beta T cells, but was enhanced by IL-2. The gamma delta T cell response proceeded in the absence of accessory cells, MHC class II, beta 2-microglobulin, or TAP-1, suggesting that Ag presentation by MHC class I/II molecules and peptide processing are not required. Among normal splenocytes, as with gamma delta T cell hybridomas, the response was strongest with V gamma 1+ gamma delta T cells, and in comparison with related polypeptides, pE50Y50 provided the strongest stimulus for these cells. TCR gene transfer into a TCR-deficient alpha beta T cell showed that besides the TCR, no other components unique to gamma delta T cells are needed. Furthermore, interactions between only the T cells and pE50Y50 were sufficient to bring about the response. Thus, pE50Y50 elicited a response distinct from those of T cells to processed/presented peptides or superantigens, consistent with a mechanism of Ig-like ligand recognition of gamma delta T cells. Direct stimulation by ligands resembling pE50Y50 may thus selectively evoke contributions of gamma delta T cells to the host response.

The expansion of human gammadelta T cells in response to Daudi cells requires the participation of CD4+ T cells

The Burkitt's lymphoma cell line Daudi is a potent inducer of human gammadelta T-cell expansion. Using an in vitro culture system comprised of irradiated Daudi cells as stimulators and normal human lymphocytes as responders, the cellular determinants of this response were investigated. Three of four monoclonal antibodies (mAbs 1-1C4, L243, and 9.3F10) directed against disparate epitopes of human major histocompatibility complex (MHC) class II, as well as a mAb with specificity for CD4 (OKT4), inhibited the expansion of gammadelta T cells in response to Daudi cell stimulators. mAbs with a specificity for CD74 and CD8 were non-inhibitory. Lymphocyte depletion experiments demonstrated a critical role for the CD4+ T-cell subset in the expansion of gammadelta T cells. Other data pointed towards requirements for direct cell contact in this system, and the addition of exogenous recombinant interleukin (IL)-2, IL-4, and IL-12 failed to reconstitute gammadelta T-cell expansion in CD4+ lymphocyte-depleted cultures. These results complement previous findings in murine infectious disease and mycobacterial systems, providing a direct demonstration that CD4+ T cells play a role in gammadelta T-cell expansion through an interaction with human leucocyte antigen (HLA) class II on Daudi cells. The data point towards important functional links between the acquired and natural immune systems.

Bovine gammadelta T-cell responses to the intracellular protozoan parasite Theileria parva

T cells bearing the gammadelta antigen receptor (gammadelta T cells) can constitute up to 50% of T cells in the peripheral blood and lymphoid organs of young cattle. We present data showing that gammadelta T cells are involved in immune responses against Theileria parva. gammadelta T cells isolated from peripheral blood mononuclear cells (PBMC) of T. parva-naive and -immune cattle proliferated in the presence of fixed or unfixed autologous T. parva-infected lymphoblasts (TpL) and heat-stressed concanavalin A (ConA)-induced blasts (ConA blasts) but not untreated ConA blasts. The specificity of response was further evaluated with a panel of gammadelta T-cell lines and clones. T-cell reactivity was blocked by GB21A, a monoclonal antibody (MAb) specific for the gammadelta T-cell receptor, but not by MAbs specific for class I and class II major histocompatibility complex (MHC) molecules. In addition, TpL but not ConA blasts from a variety of MHC-mismatched animals induced proliferation of the gammadelta T-cell lines and clones. These gammadelta T cells were found to respond to TpL infected with several different parasite stocks and failed to recognize TpL after elimination of the parasite by the theilericidal drug BW 720C. Assays for cytotoxic activity of gammadelta T cells sorted from bulk cultures of immune PBMC restimulated several times with autologous TpL demonstrated that effector cells whose specificity is similar to that of proliferating cells are generated. These results suggest that bovine gammadelta T cells are activated by and lyse T. parva-infected cells by recognizing conserved parasite-induced or parasite-derived antigens in an MHC-unrestricted fashion.

Cell surface expression of HLA-E: interaction with human beta2-microglobulin and allelic differences

The formation of a trimeric complex composed of MHC class I heavy chain, beta2-microglobulin (beta2m) and peptide ligand is a prerequisite for its efficient transport to the cell surface. We have previously demonstrated impaired intracellular transport of the human class Ib molecule HLA-E in mouse myeloma X63 cells cotransfected with the genes for HLA-E and human beta2m (hbeta2m), which is most likely attributable to inefficient intracellular peptide loading of the HLA-E molecule. Here we demonstrate that cell surface expression of HLA-E in mouse cells strictly depends on the coexpression of hbeta2m and that soluble empty complexes of HLA-E and hbeta2m display a low degree of thermostability. Both observations imply that low affinity interaction of HLA-E with beta2m accounts to a considerable extent for the observed low degree of peptide uptake in the endoplasmic reticulum. Moreover, we show that the only allelic variation present in the caucasoid population located at amino acid position 107 (Gly or Arg) greatly affects intracellular transport and cell surface expression upon transfection of the respective alleles into mouse cells. No obvious difference was found with regard to the sequence of the peptide ligand.

Cross-talk between gammadelta T lymphocytes and immune cells in humoral response

The role of gamma delta T cells in immunoregulation is largely unknown. In the current study we noted that gamma delta T cells play a positive role in the humoral response. These positively acting gamma delta T cells are required for the successful adoptive cell transfer of the humoral response, as well as for in vitro generation of plaque-forming cells (PFC). The presented results show that gammadelta T cells cause an increase in interleukin-10 (IL-10) production, which partly elucidates the mechanism of action of these cells. However, experiments with cell culture inserts strongly suggest that direct cell-cell contact between immune and gamma delta H-2-compatible regulatory T cells is critical to the exertion of the positive immunoregulatory function of gamma delta cells. The mechanism of cross-talk between these two cell populations is still not clear but we regard as most likely that the positively acting gamma delta T cells may interact with a complex of heat-shock protein-non-polymorphic MHC (IB) on the surface of T helper type 2 and/or B cells. This could provide, by direct cell-cell contact, the cognate recognition between gamma delta T-cell receptors and heat-shock protein-MHC that leads to positive internal signalling in the immune cells.

Down-Regulation of Tumoricidal NK and NK T Cell Activities by MHC Kb Molecules Expressed on Th2-Type γδ T and αβ T Cells Coinfiltrating in Early B16 Melanoma Lesions

We examined whether γδ T and αβ T cells accumulating in early B16 melanoma lesions regulate NK and NK T cells that attack tumor cells. Freshly isolated and cultured tumor-infiltrating lymphocyte (TIL) populations of NK and NK T cells lysed B16 and produced IFN-γ, whereas γδ T and a large part of αβ T cell populations had no substantial cytotoxicity against B16 and secreted Th2 cytokines. Furthermore, the freshly isolated NK1.1+ TIL population exhibited a higher anti-B16 effect than did splenocytes. γδ T and αβ T cell populations dramatically inhibited the cytotoxicity of NK and NK T cells in an MHC Kb-dependent manner. Culture supernatant from γδ T and αβ T cell populations inhibited the proliferation of NK and NK T cell populations but did not affect their cytotoxicity, suggesting that the released Th2 cytokines are merely partly involved in the down-modulation of NK-lineage cells. NK1.1+ cells obtained from TIL of γδ T cell-depleted mice significantly lysed B16 cells compared with those from control mice. Finally, anti-Kb Fab mAb injected intralesionally at an early, but not at a late, stage of development of B16 melanoma inhibited tumor growth. These findings suggest that Th2-type γδ T and αβ T cells infiltrating in early B16 development inhibit the tumoricidal activity of NK-lineage cells using their class I molecules and partly their suppressive cytokines.

Transporters associated with antigen processing (TAP)-independent presentation of soluble insulin to alpha/beta T cells by the class Ib gene product, Qa-1(b)

T cell hybridomas isolated from nonresponder H-2(b) mice immunized with pork insulin were stimulated by insulin in the presence of major histocompatibility complex (MHC)-unmatched antigen presenting cells. The restriction element used by these CD4(-) T cells was mapped to an oligomorphic MHC class Ib protein encoded in the T region and identified as Qa-1(b) using transfectants. The antigenic determinant was localized to the insulin B chain, and experiments with truncated peptides suggested that it is unexpectedly long, comprising most or all of the 30 amino acid B chain. The antigen processing pathway used to present insulin to the Qa-1(b)- restricted T cells does not require transporters associated with antigen processing (TAP), and it is inhibited by chloroquine. A wide variety of cell lines from different tissues efficiently present soluble insulin to Qa-1(b)-restricted T cells, and insulin presentation is not enhanced by phagocytic stimuli. Our results demonstrate that Qa-1(b) can function to present exogenous protein to T cells in a manner similar to MHC class II molecules. Therefore, this class Ib protein may have access to a novel antigen processing pathway that is not available to class Ia molecules.

Structure and function of the human MHC class Ib molecules HLA-E, HLA-F and HLA-G

The major histocompatibility (MHC) class Ib molecules HLA-E, HLA-F and HLA-G are relatively non-polymorphic compared to class Ia molecules. Both HLA-E and HLA-G bind peptides and are involved in natural killer (NK)-cell recognition, but the role of HLA-F is unclear. HLA-E binds specifically to the conserved leader sequence peptides from the class Ia MHC molecules and interacts on the cell surface with the CD94/NKG2 class of NK-cell receptors. The framework structure of HLA-E is similar to that of the MHC class Ia molecules, but the peptide-binding groove is highly adapted for the specific binding of the leader sequence peptides. This is different from class Ia molecules, which have highly promiscuous peptide-binding grooves. The HLA-E groove makes full use of all the available pockets and imposes specificity along the entire length of the peptide. HLA-G binds nonamer peptides with leucine or isoleucine at position 2, proline at position 3 and leucine at position 9. Expression of HLA-G inhibits NK cells expressing the CD94/NKG2 class of receptors, though an interaction with these receptors has not been directly demonstrated.

Interaction of HLA-E with Peptides and the Peptide Transporter In Vitro: Implications for its Function in Antigen Presentation

The assembly of MHC Ia molecules in the endoplasmic reticulum requires the presence of peptide ligands and β2m and is facilitated by chaperones in an ordered sequence of molecular interactions. A crucial step in this process is the interaction of the class I α-chain/β2m dimer with TAP, which is believed to ensure effective peptide loading of the empty class I molecule. We have previously demonstrated impaired intracellular transport of the class Ib molecule HLA-E in mouse myeloma cells cotransfected with the genes for HLA-E and human β2m, which is most likely attributable to inefficient intracellular peptide loading of the HLA-E molecule. We therefore analyzed the ability of HLA-E in the transfectant cell line to bind synthetic peptides by means of their ability to enhance cell surface expression of HLA-E. Peptide binding was confirmed by testing the effect on the thermostability of soluble empty HLA-E/human β2m dimers. Two viral peptides binding to HLA-E were thus identified, for which the exact positioning of the N terminus appeared critical for binding, whereas the contribution of the length of the C terminus seemed to be minor, allowing peptides as short as seven amino acids and up to 16 amino acids to exhibit considerable binding activity. Furthermore, we demonstrate that HLA-E interacts with TAP and that this interaction can be prolonged by the proteasome inhibitor N-acetyl-l-leucyl-l-leucyl-l-norleucinal, which reduces the intracellular peptide pool. The presented data indicate that HLA-E is capable of presenting peptide ligands similar to the repertoire of HLA class Ia molecules.

Structural features impose tight peptide binding specificity in the nonclassical MHC molecule HLA-E

The crystal structure of the nonclassical human class lb MHC molecule HLA-E has been determined in complex with a prototypic ligand, the nonamer peptide (VMAPRTVLL), derived from the highly conserved residues 3-11 of the human MHC class la leader sequence. The mode of peptide binding retains some of the standard features observed in MHC class la complexes, but novel features imply that HLA-E has evolved to mediate specific binding to a tightly defined set of almost identical hydrophobic peptides from the highly conserved class l leader sequences. These molecular adaptations make HLA-E a rigorous checkpoint at the cell surface reporting on the integrity of the antigen processing pathway to CD94/NKG2 receptor-bearing natural killer cells.

Gamma delta cells regulate autoimmunity

Gamma delta cells are attractive candidates for mediators of autoimmune disease. They can expand in germ-free mice, probably through recognition of autoantigens, and gamma delta-cell-deficient mice, unlike mice deficient in alpha beta T cells or B cells, show no severe defects in the immune response to foreign antigen challenge. A capacity of gamma delta cells to effect or regulate tissue damage is also plausible, given their ready localization to tissues, and their myriad of effector functions. Added to this, attempts to reconstruct the physiological course of autoimmune diseases with only autoreactive alpha beta T cells seem invariably to fall short for lack of other unidentified players. Gamma delta cells and their putative ligands have been linked to autoimmune conditions, and recent experiments confirm that gamma delta cells play a significant role in autoimmune disease in vivo.

Qa-1 interaction and T cell recognition of the Qa-1 determinant modifier peptide

The peptide-binding properties of the nonclassical major histocompatibility complex (MHC) class 1b molecule Qa-1 were investigated using a transfected hybrid molecule composed of the alpha 1 and alpha 2 domains of Qa-1b and the alpha 3 domain of H-2Db. This allowed the use of a monoclonal antibody directed against H-2Db whilst retaining the peptide-binding groove of Qa-1b. By comparison with classical MHC class I molecules, intracellular maturation of the chimeric molecule was inefficient with weak intracellular association with beta 2-microglobulin. However, at the cell surface the hybrid molecules were stably associated with beta 2-microglobulin and were recognized by cytotoxic T lymphocyte (CTL) clones specific for the Qa-1b-presented peptide Qdm (AMAPRTLLL). A whole-cell binding assay was used to determine which residues of Qdm were important for binding to Qa-1b and CTL clones served to identify residues important for T cell recognition. Substitutions at position 1 and 5 did not reduce the efficiency of binding and had little effect on CTL recognition. In contrast, substitutions at position 9 resulted in loss of MHC class I binding. Mass spectrometric analysis of peptides eluted from immunopurified Qa-1b/Db molecules indicated that Qdm was the dominant peptide. The closely related peptide, AMVPRTLLL, which is derived from the signal sequence of H-2Dk, was also present, although it was considerably less abundant. The mass profile suggested the presence of additional peptides the majority of which consisted of eight to ten amino acid residues. Finally, the finding that a peptide derived from Klebsiella pneumoniae can bind raises the possibility that this non-classical MHC class I molecule may play a role in the presentation of peptides of microorganisms.

The T cell receptor repertoire of intestinal intraepithelial gammadelta T lymphocytes is influenced by genes linked to the major histocompatibility complex and to the T cell receptor loci

Most of the gammadelta T cells in the intestinal epithelium of normal mice use the Vgamma1 or the Vgamma7 gene segments. However, the relative proportions of gammadelta intraepithelial lymphocytes expressing either the Vgamma1 or the Vgamma7 chain vary among different strains of mice whereas they are quite constant between different individuals of the same strain, suggesting that genetic factors, rather than environmental factors, are responsible for the observed differences. To analyze the genetic factors influencing the representation of different gammadelta T cell subsets in the intestinal epithelium, we used available anti-T cell antigen receptor (TCR) V region-specific mAbs against Vgamma1, Vgamma4, Vgamma7, and Vdelta4 to examine the TCR repertoire of intraepithelial gammadelta lymphocytes in a set of (C57BL/6 x DBA/2) recombinant inbred strains. Our results show that the representation of different Vgamma and Vdelta gene products among gammadelta intestinal intraepithelial lymphocytes is under a complex genetic control with a marked influence by genes closely linked to the TCRgamma, TCRdelta, and major histocompatibility complex loci.

Donor γδ T Lymphocytes Promote Allogeneic Engraftment Across the Major Histocompatibility Barrier in Mice

T cells that express the αβ T-cell receptor are thought to be the T-cell population primarily responsible for facilitating alloengraftment. The role of γδ+ T cells that comprise only a minority of mature T cells in promoting allogeneic engraftment, however, has not been extensively studied. The purpose of this study was to determine whether γδ T cells were capable of facilitating alloengraftment in murine recipients of major histocompatibility complex-mismatched marrow grafts. We developed a model where engraftment of C57BL/6 × 129/F2 (H-2b) marrow in sublethally irradiated (800 cGy) recipients (AKR/J, H-2k) is dependent on the presence of mature donor T cells in the marrow graft. In this model, donor T-cell engraftment was significantly augmented by as few as 1 × 105 αβ T cells. The role of γδ T cells was then investigated using transgenic donors (C57BL/6 × 129 background) in which a portion of the T-cell receptor–β chain gene was deleted by gene targeting so that these mice lack αβ T cells. Addition of 10 × 106 naive γδ T cells to T-cell depleted marrow grafts was required to significantly increase alloengraftment, although donor T cells averaged <50% of total splenic T cells. To determine whether higher doses of γδ T cells would improve donor engraftment and eradicate residual host T cells, γδ T cells were ex vivo expanded with a γδ T-cell–specific monoclonal antibody and interleukin-2 and then transplanted into irradiated recipients. Transplantation of ≥ 160 × 106 activated γδ T cells was necessary to consistently and significantly augment donor cell chimerism and enhance hematopoietic reconstitution when compared to control mice, but host T cells persisted in these chimeras. Addition of 2.5 × 104 mature αβ T cells, which alone were incapable of facilitating engraftment, to T-cell depleted marrow grafts containing 160 × 106 activated γδ T cells resulted in long-term (<100 day) complete donor engraftment, indicating that limiting numbers of αβ T cells were required in the marrow graft for the eradication of residual host T cells. Using serial weight curves and B-cell reconstitution as end points, clinically significant graft-versus-host disease was not observed in these chimeras under these experimental conditions. These data show that, whereas less potent than αβ T cells, γδ T cells are able to promote engraftment and enhance hematopoietic reconstitution in allogeneic marrow transplant recipients.

Gamma delta T cells: their immunobiology and role in malaria infections

The status of research on gamma delta T cells is reviewed. Recent research shows that gamma delta T cells may see antigens in an immunoglobulin-like manner and that non-peptidic substance can be antigens for these cells. Considerable advances have been made in defining the immunobiology of gamma delta T cells, with evidence for sentinel, protective and immunoregulatory roles. Research on gamma delta T cells in malaria infections suggests that gamma delta T cells are mediators of protective immunity, most probably through the production of Th1 cytokines such as TNF alpha, TNF delta and IFN gamma and that excessive production of such cytokines may contribute to pathology. Our data on the features of the peripheral blood gamma delta T cells response in humans infected with Plasmodium falciparum show that there is considerable variation between individuals in the relative expansion of gamma delta T lymphocytes following primary or secondary infection. They confirm that activation of gamma delta T cells occurs during P. falciparum infection and that activated cells can persist for many weeks after treatment. The possibility that gamma delta T cells have an immunoregulatory function in malaria infections is proposed.