CD4+ T cell help improves CD8+ T cell memory by retained CD27 expression

CD4+ T cell help during the priming of CD8+ T lymphocytes imprints the capacity for optimal secondary expansion upon re-encounter with antigen. Helped memory CD8+ T cells rapidly expand in response to a secondary antigen exposure, even in the absence of T cell help and, are most efficient in protection against a re-infection. In contrast, helpless memory CTL can mediate effector function, but secondary expansion is reduced. How CD4+ T cells instruct CD8+ memory T cells during priming to undergo efficient secondary expansion has not been resolved in detail. Here, we show that memory CTL after infection with lymphocytic choriomeningitis virus are CD27(high) whereas memory CTL primed in the absence of CD4+ T cell have a reduced expression of CD27. Helpless memory CTL produced low amounts of IL-2 and did not efficiently expand after restimulation with peptide in vitro. Blocking experiments with monoclonal antibodies and the use of CD27(-/-) memory CTL revealed that CD27 ligation during restimulation increased autocrine IL-2 production and secondary expansion. Therefore, regulating CD27 expression on memory CTL is a novel mechanism how CD4+ T cells control CTL memory.

Stimulation via CD40 can substitute for CD4 T cell function in preventing reactivation of a latent herpesvirus

Reactivation of latent herpesviruses is a particular problem in immunocompromised individuals, such as AIDS patients, who lack effective CD4 T helper cell function. An important question is whether residual immune defenses can be mobilized to combat such opportunistic infections, in the absence of CD4 T cells. In the present study, we used a mouse model of opportunistic infection to determine whether stimulation via CD40 could substitute for CD4 T cell function in preventing reactivation of a latent herpesvirus. Treatment with an agonistic antibody to CD40 was highly effective in preventing reactivation of latent murine gammaherpesvirus (MHV-68) in the lungs of CD4 T cell-deficient mice. CD8+ T cells were essential for this effect, whereas virus-specific serum antibody was undetectable and IFN-γ production was unchanged. This demonstration that immunostimulation via CD40 can replace CD4 T cell help in controlling latent virus in vivo has potential implications for the development of novel therapeutic agents to prevent viral reactivation in immunocompromised patients.

CD4+ T cell recognition of MHC class II-restricted epitopes from NY-ESO-1 presented by a prevalent HLA DP4 allele: Association with NY-ESO-1 antibody production

NY-ESO-1 is a tumor-specific shared antigen with distinctive immunogenicity. Both CD8+ T cells and class-switched Ab responses have been detected from patients with cancer. In this study, a CD4+ T cell line was generated from peripheral blood mononuclear cells of a melanoma patient and was shown to recognize NY-ESO-1 peptides presented by HLA-DP4, a dominant MHC class II allele expressed in 43–70% of Caucasians. The ESO p157–170 peptide containing the core region of DP4-restricted T cell epitope was present in a number of tumor cell lines tested and found to be recognized by both CD4+ T cells as well as HLA-A2-restricted CD8+ T cells. Thus, the ESO p157–170 epitope represents a potential candidate for cancer vaccines aimed at generating both CD4+ and CD8+ T cell responses. More importantly, 16 of 17 melanoma patients who developed Ab against NY-ESO-1 were found to be HLA-DP4-positive. CD4+ T cells specific for the NY-ESO-1 epitopes were generated from 5 of 6 melanoma patients with NY-ESO-1 Ab. In contrast, no specific DP4-restricted T cells were generated from two patients without detectable NY-ESO-1 Ab. These results suggested that NY-ESO-1-specific DP4-restricted CD4+ T cells were closely associated with NY-ESO-1 Ab observed in melanoma patients and might play an important role in providing help for activating B cells for NY-ESO-1-specific Ab production.

Tissue requirements for establishing long-term CD4+ T cell-mediated immunity following Leishmania donovani infection

Organ-specific immunity is a feature of many infectious diseases, including visceral leishmaniasis caused by Leishmania donovani. Experimental visceral leishmaniasis in genetically susceptible mice is characterized by an acute, resolving infection in the liver and chronic infection in the spleen. CD4+ T cell responses are critical for the establishment and maintenance of hepatic immunity in this disease model, but their role in chronically infected spleens remains unclear. In this study, we show that dendritic cells are critical for CD4+ T cell activation and expansion in all tissue sites examined. We found that FTY720-mediated blockade of T cell trafficking early in infection prevented Ag-specific CD4+ T cells from appearing in lymph nodes, but not the spleen and liver, suggesting that early CD4+ T cell priming does not occur in liver-draining lymph nodes. Extended treatment with FTY720 over the first month of infection increased parasite burdens, although this associated with blockade of lymphocyte egress from secondary lymphoid tissue, as well as with more generalized splenic lymphopenia. Importantly, we demonstrate that CD4+ T cells are required for the establishment and maintenance of antiparasitic immunity in the liver, as well as for immune surveillance and suppression of parasite outgrowth in chronically infected spleens. Finally, although early CD4+ T cell priming appeared to occur most effectively in the spleen, we unexpectedly revealed that protective CD4+ T cell-mediated hepatic immunity could be generated in the complete absence of all secondary lymphoid tissues.

Rapid burst of H2O2 by plant growth regulators increases intracellular Ca2+ amounts and modulates CD4(+) T cell activation

The identification of small molecules that affect T cell activation is an important area of research. Three molecules that regulate plant growth and differentiation, but not their structurally similar analogs, were identified to enhance primary mouse CD4(+) T cell activation in conjunction with soluble anti-CD3 stimulation: Indoleacetic acid (natural plant auxin), 1-Napthaleneacetic acid (synthetic plant auxin) and 2,4-Dichlorophenoxyacetic acid (synthetic plant auxin and herbicide). These effects are distinct in comparison to Curcumin, the well known phenolic immunomodulator, which lowers T cell activation. An investigation into the mechanisms of action of the three plant growth regulators revealed a rapid induction of reactive oxygen species (ROS), mainly comprising H2O2 . In addition, these three molecules synergize with soluble anti-CD3 signaling to enhance intracellular Ca2+ concentrations Ca2+](i), leading to greater T cell activation, e.g. induction of CD25 and IL-2. Enhanced production of TNF alpha and IFN gamma by CD4+ T cells is also observed upon plant growth regulator treatment with soluble anti-CD3. Interestingly, maximal IL-2 production and CD4(+) T cell cycle progression are observed upon activation with soluble anti-CD3 and phorbol 12-myristate 13-acetate (PMA), a phorbol ester. Additionally, stimulation with PMA and Ionomcyin (a Ca2+ ionophore), which activates T cells by circumventing the TCR, and plant growth regulators also demonstrated the role of the strength of signal (SOS): T cell cycle progression is enhanced with gentle activation conditions but decreased with strong activation conditions. This study demonstrates the direct effects of three plant growth regulators on CD4(+) T cell activation and cycling. (C) 2010 Elsevier B.V. All rights reserved.

Metabolic programming and PDHK1 control CD4+ T cell subsets and inflammation.

Activation of CD4+ T cells results in rapid proliferation and differentiation into effector and regulatory subsets. CD4+ effector T cell (Teff) (Th1 and Th17) and Treg subsets are metabolically distinct, yet the specific metabolic differences that modify T cell populations are uncertain. Here, we evaluated CD4+ T cell populations in murine models and determined that inflammatory Teffs maintain high expression of glycolytic genes and rely on high glycolytic rates, while Tregs are oxidative and require mitochondrial electron transport to proliferate, differentiate, and survive. Metabolic profiling revealed that pyruvate dehydrogenase (PDH) is a key bifurcation point between T cell glycolytic and oxidative metabolism. PDH function is inhibited by PDH kinases (PDHKs). PDHK1 was expressed in Th17 cells, but not Th1 cells, and at low levels in Tregs, and inhibition or knockdown of PDHK1 selectively suppressed Th17 cells and increased Tregs. This alteration in the CD4+ T cell populations was mediated in part through ROS, as N-acetyl cysteine (NAC) treatment restored Th17 cell generation. Moreover, inhibition of PDHK1 modulated immunity and protected animals against experimental autoimmune encephalomyelitis, decreasing Th17 cells and increasing Tregs. Together, these data show that CD4+ subsets utilize and require distinct metabolic programs that can be targeted to control specific T cell populations in autoimmune and inflammatory diseases.

Cutting edge: distinct glycolytic and lipid oxidative metabolic programs are essential for effector and regulatory CD4+ T cell subsets.

Stimulated CD4(+) T lymphocytes can differentiate into effector T cell (Teff) or inducible regulatory T cell (Treg) subsets with specific immunological roles. We show that Teff and Treg require distinct metabolic programs to support these functions. Th1, Th2, and Th17 cells expressed high surface levels of the glucose transporter Glut1 and were highly glycolytic. Treg, in contrast, expressed low levels of Glut1 and had high lipid oxidation rates. Consistent with glycolysis and lipid oxidation promoting Teff and Treg, respectively, Teff were selectively increased in Glut1 transgenic mice and reliant on glucose metabolism, whereas Treg had activated AMP-activated protein kinase and were dependent on lipid oxidation. Importantly, AMP-activated protein kinase stimulation was sufficient to decrease Glut1 and increase Treg generation in an asthma model. These data demonstrate that CD4(+) T cell subsets require distinct metabolic programs that can be manipulated in vivo to control Treg and Teff development in inflammatory diseases.

Antigen-specific T-T interactions regulate CD4 T-cell expansion.

The regulation of CD4 T cell numbers during an immune response should take account of the amount of antigen (Ag), the initial frequency of Ag-specific T cells, the mix of naive versus experienced cells, and (ideally) the diversity of the repertoire. Here we describe a novel mechanism of T cell regulation that potentially deals with all of these parameters. We found that CD4 T cells establish a negative feedback loop by capturing their cognate MHC/peptide complexes from Ag-presenting cells and presenting them to Ag-experienced CD4 T cells, thereby inhibiting their recruitment into the response while allowing recruitment of naive T cells. The inhibition is Ag specific, begins at day 2 (long before Ag disappearance), and cannot be overcome by providing new Ag-loaded dendritic cells. In this way CD4 T cell proliferation is regulated in a functional relationship to the amount of Ag, while allowing naive T cells to generate repertoire variety.

Repertoire of HLA-DR1-restricted CD4 T cell responses to capsular Caf1 antigen of Y. pestis HLA-transgenic mice

Yersinia pestis is the causative agent of plague, a rapidly fatal infectious disease that has not been eradicated worldwide. The capsular Caf1 protein of Y. pestis is a protective antigen under development as a recombinant vaccine. However, little is known about the specificity of human T cell responses for Caf1. We characterized CD4 T cell epitopes of Caf1 in 'humanized'-HLA-DR1 transgenic mice lacking endogenous MHC class II molecules. Mice were immunized with Caf1 or each of a complete set of overlapping synthetic peptides, and CD4 T cell immunity was measured with respect to proliferative and IFNgamma T cell responses and recognition by a panel of T cell hybridomas, as well as direct determination of binding affinities of Caf1 peptides to purified HLA-DR molecules. Although a number of DR1-restricted epitopes were identified following Caf1 immunization, the response was biased towards a single immunodominant epitope near the C-terminus of Caf1. In addition, potential promiscuous epitopes, including the immunodominant epitope, were identified by their ability to bind multiple common HLA alleles, with implications for the generation of multivalent vaccines against plague for use in humans.

Cell-to-Cell Interactions and Signals Involved in the Reconstitution of Peripheral CD8(+) T-CM and T-EM Cell Pools

We here describe novel aspects of CD8(+) and CD4(+) T cell subset interactions that may be clinically relevant and provide new tools for regulating the reconstitution of the peripheral CD8(+) T cell pools in immune-deficient states. We show that the reconstitution capacity of transferred isolated naive CD8(+) T cells and their differentiation of effector functions is limited, but both dramatically increase upon the co-transfer of CD4(+) T cells. This helper effect is complex and determined by multiple factors. It was directly correlated to the number of helper cells, required the continuous presence of the CD4(+) T cells, dependent on host antigen-presenting cells (APCs) expressing CD40 and on the formation of CD4/CD8/APC cell clusters. By comparing the recovery of (CD44(+)CD62L(high)) T-CM and (CD44(+)CD62L(low)) T-EM CD8(+) T cells, we found that the accumulation of TCM and TEM subsets is differentially regulated. T-CM-cell accumulation depended mainly on type I interferons, interleukin (IL)-6, and IL-15, but was independent of CD4(+) T-cell help. In contrast, TEM-cell expansion was mainly determined by CD4(+) T-cell help and dependent on the expression of IL-2R beta by CD8 cells, on IL-2 produced by CD4(+) T-cells, on IL-15 and to a minor extent on IL-6.

Anthrax lethal toxin and the induction of CD4 T cell immunity

Bacillus anthracis secretes exotoxins which act through several mechanisms including those that can subvert adaptive immunity with respect both to antigen presenting cell and T cell function. The combination of Protective Antigen (PA) and Lethal Factor (LF) forming Lethal Toxin (LT), acts within host cells to down-regulate the mitogen activated protein kinase (MAPK) signaling cascade. Until recently the MAPK kinases were the only known substrate for LT; over the past few years it has become evident that LT also cleaves Nlrp1, leading to inflammasome activation and macrophage death. The predicted downstream consequences of subverting these important cellular pathways are impaired antigen presentation and adaptive immunity. In contrast to this, recent work has indicated that robust memory T cell responses to B. anthracis antigens can be identified following natural anthrax infection. We discuss how LT affects the adaptive immune response and specifically the identification of B. anthracis epitopes that are both immunogenic and protective with the potential for inclusion in protein sub-unit based vaccines.

Dynamics of HIV latency and reactivation in a primary CD4+ T cell model.

HIV latency is a major obstacle to curing infection. Current strategies to eradicate HIV aim at increasing transcription of the latent provirus. In the present study we observed that latently infected CD4+ T cells from HIV-infected individuals failed to produce viral particles upon ex vivo exposure to SAHA (vorinostat), despite effective inhibition of histone deacetylases. To identify steps that were not susceptible to the action of SAHA or other latency reverting agents, we used a primary CD4+ T cell model, joint host and viral RNA sequencing, and a viral-encoded reporter. This model served to investigate the characteristics of latently infected cells, the dynamics of HIV latency, and the process of reactivation induced by various stimuli. During latency, we observed persistence of viral transcripts but only limited viral translation. Similarly, the reactivating agents SAHA and disulfiram successfully increased viral transcription, but failed to effectively enhance viral translation, mirroring the ex vivo data. This study highlights the importance of post-transcriptional blocks as one mechanism leading to HIV latency that needs to be relieved in order to purge the viral reservoir.