Arati B. Kamath

Antigens in tea-beverage prime human Vγ2Vδ2 T cells in vitro and in vivo for memory and nonmemory antibacterial cytokine responses

Human γδ T cells mediate innate immunity to microbes via T cell receptor-dependent recognition of unprocessed antigens with conserved molecular patterns. These nonpeptide alkylamine antigens are shared by tumor cells, bacteria, parasites, and fungi but also by edible plant products such as tea, apples, mushrooms, and wine. Here we show that priming of γδ T cells with alkylamine antigens in vitro results in a memory response to these antigens. Such priming results also in a nonmemory response to whole bacteria and to lipopolysaccharide, characterized by IL-12-dependent secretion of IFN-γ by γδ T cells and by γδ T cell proliferation. Drinking tea, which contains l-theanine, a precursor of the nonpeptide antigen ethylamine, primed peripheral blood γδ T cells to mediate a memory response on reexposure to ethylamine and to secrete IFN-γ in response to bacteria. This unique combination of innate immune response and immunologic memory shows that γδ T cells can function as a bridge between innate and acquired immunity. In addition, these data provide an explanation for the health benefits of tea.

Cytolytic CD8 T Cells Recognizing CFP10 Are Recruited to the Lung after Mycobacterium tuberculosis Infection

Optimum immunity against Mycobacterium tuberculosis requires both CD4+ and CD8+ T cells. In contrast with CD4+ T cells, few antigens are known that elicit CD8+ T cells during infection. CD8+ T cells specific for culture filtrate protein-10 (CFP10) are found in purified protein derivative positive donors, suggesting that CFP10 primes CD8+ T cells in vivo. Using T cells from M. tuberculosis–infected mice, we identified CFP10 epitopes recognized by CD8+ T cells and CD4+ T cells. CFP10-specific T cells were detected as early as week 3 after infection and at their peak accounted for up to 30% of CD8+ T cells in the lung. IFNγ-producing CD8+ and CD4+ T cells recognizing CFP10 epitopes were preferentially recruited to the lungs of M. tuberculosis–infected mice. In vivo cytolytic activity of CD8+ T cells specific for CFP10 and TB10.3/10.4 proteins was detected in the spleen, pulmonary lymph nodes, and lungs of infected mice. The cytolytic activity persisted long term and could be detected 260 d after infection. This paper highlights the cytolytic function of antigen-specific CD8+ T cells elicited by M. tuberculosis infection and demonstrates that large numbers of CFP10-specific cytolytic CD8+ T cells are recruited to the lung after M. tuberculosis infection.

Antigen-Specific CD8+ T Cells and the Development of Central Memory during Mycobacterium tuberculosis Infection

Whether true memory T cells develop in the face of chronic infection such as tuberculosis remains controversial. To address this question, we studied CD8+ T cells specific for the Mycobacterium tuberculosis ESAT6-related Ags TB10.3 and TB10.4. The shared epitope TB10.3/10.420–28 is presented by H-2 Kd, and 20–30% of the CD8+ T cells in the lungs of chronically infected mice are specific for this Ag following respiratory infection with M. tuberculosis. These TB10.3/10.420–28-specific CD8+ T cells produce IFN-γ and TNF and express CD107 on their cell surface, which indicates their likely role as CTL in vivo. Nearly all of the Ag-specific CD8+ T cells in the lungs of chronically infected mice had a T effector cell phenotype based on their low expression of CD62L and CD45RB. In contrast, a population of TB10.3/10.420–28-specific CD8+ T cells was identified in the lymphoid organs that express high levels of CD62L and CD45RB. Antibiotic treatment to resolve the infection led to a contraction of the Ag-specific CD8+ T cell population and was accompanied by an increase in the proportion of CD8+ T cells with a central memory phenotype. Finally, challenge of memory-immune mice with M. tuberculosis was accompanied by significant expansion of TB10.3/10.420–28-specific CD8+ T cells, which suggests that these cells are in fact functional memory T cells.

Toll-Like Receptor 4-Defective C3H/HeJ Mice Are Not More Susceptible than Other C3H Substrains to Infection with Mycobacterium tuberculosis

ABSTRACT Mycobacterium tuberculosis produces a variety of molecules capable of activating Toll-like receptors, a family of pattern recognition receptors expressed by macrophages and a variety of other cells. To determine whether Toll-like receptor 4 (TLR4) was critical in resistance to M. tuberculosis infection, we compared the morbidity and mortality of TLR4-defective C3H/HeJ mice to those of TLR4-sufficient C3H mouse substrains. TLR4-defective C3H/HeJ mice and TLR4-sufficient C3H/HeSnJ, C3HeB/FeJ, and C3H/HeOuJ mice were infected by the aerosol route with M. tuberculosis. TLR4-defective C3H/HeJ mice had levels of cytokines in their bronchoalveolar lavage fluids and in vitro mycobacterial antigen-specific recall responses similar to those of other C3H mouse substrains. In addition, bacterial replication and long-term survival of mice following infection appeared to be independent of TLR4. Interestingly, C3HeB/FeJ mice were significantly more susceptible to M. tuberculosis infection, indicating that genetic heterogeneity among inbred C3H mouse substrains modifies resistance to infection. Therefore, cautious interpretation is required when the C3H/HeJ strain is used as a model of a TLR4-defective mouse strain, as there are significant allelic differences between C3H/HeJ and other C3H mouse substrains in response to M. tuberculosis infection. With this caveat, our data indicate that TLR4 may not be required for optimal immunity of mice to M. tuberculosis.

Human Vγ2Vδ2 T Cells Produce IFN-γ and TNF-α with an On/Off/On Cycling Pattern in Response to Live Bacterial Products

Whereas cytokine production in αβ T cells is rapidly regulated by exposure to peptide Ag, the mechanisms regulating cytokine production by γδ T cells are unknown. In this study, we demonstrate that human Vγ2Vδ2 T cells produce IFN-γ and TNF-α as early as 2 h after Ag exposure, and that they produce these cytokines in a dose- and time- dependent manner in response to stimulation with a live bacterial product, iso-butylamine (IBA), but not to dead bacteria or LPS. γδ T cells began, ceased, and then resumed IFN-γ and TNF-α generation in an on/off/on cycling pattern, both in vitro and in vivo, depending on the presence or absence of IBA. IFN-γ and TNF-α, whose optimum production was dependent on IBA-stimulated γδ T cells, were critical for monocyte-mediated killing of Escherichia coli. By limiting cytokine production to periods of direct contact with live bacteria, γδ T cells focus their resources at the site of infection, while limiting systemic immunopathology. Thus, human γδ T cells may mediate innate resistance to extracellular bacteria via tightly regulated cytokine production without necessarily expanding in number.

The Major Histocompatibility Complex Haplotype Affects T-Cell Recognition of Mycobacterial Antigens but Not Resistance to Mycobacterium tuberculosis in C3H Mice

ABSTRACT Both innate and adaptive immunity play an important role in host resistance to Mycobacterium tuberculosis infection. Although several studies have suggested that the major histocompatibility complex (MHC) haplotype affects susceptibility to infection, it remains unclear whether the modulation of T-cell immunity by the MHC locus determines the hosts susceptibility to tuberculosis. To determine whether allelic differences in the MHC locus affect the T-cell immune response after M. tuberculosis infection, we infected inbred and H-2 congenic mouse strains by the respiratory route. The H-2 locus has a profound effect on the antigen-specific CD4+-T-cell response after M. tuberculosis infection. CD4+ T cells from infected mice of the H-2b haplotype produced more gamma interferon (IFN-γ) after in vitro stimulation with mycobacterial antigens than mice of the H-2k haplotype. A higher level of IFN-γ was also detected in bronchoalveolar lavage fluid from infected mice of the H-2b haplotype. Furthermore, C3.SW-H2b/SnJ mice generate and recruit activated T cells to the lung after infection. Despite a robust immune response, C3.SW-H2b/SnJ mice succumbed to infection early and were similarly susceptible to infection as other C3H (H-2k) substrains. These results suggest that although the MHC haplotype has a profound impact on the T-cell recognition of M. tuberculosis antigens, the susceptibility of C3H mice to infection is MHC independent.

Anamnestic Responses of Mice following Mycobacterium tuberculosis Infection

ABSTRACT The anamnestic response is the property of the immune system that makes vaccine development possible. Although the development of a vaccine against Mycobacterium tuberculosis is an important global priority, there are many gaps in our understanding of how immunological memory develops following M. tuberculosis infection or after BCG vaccination. In experiments designed to compare the anamnestic response of susceptible and resistant mouse strains, major histocompatibility complex-matched memory-immune C3.SW-H2b/SnJ and C57BL/6 mice both demonstrated better control of bacterial replication following reinfection with M. tuberculosis than control mice. Nevertheless, this memory response did not appear to have any long-term protective effect for either mouse strain. A greater understanding of the immunological factors that govern the maintenance of immunological memory following exposure to M. tuberculosis will be required to develop an effective vaccine.

Human Vγ2Vδ2 T Cells Augment Migration-Inhibitory Factor Secretion and Counteract the Inhibitory Effect of Glucocorticoids on IL-1β and TNF-α Production

In immune cells, proinflammatory cytokine gene expression is regulated by glucocorticoids, whereas migration-inhibitory factor (MIF), a pleiotropic cytokine, has the unique property of counteracting the inhibitory effect of glucocorticoids on TNF-α and IL-1β secretion. A few lines of evidence suggest that γδ T cells play an important role in immunoregulation. However, it is unknown whether human γδ T cells participate in regulating MIF secretion, and how γδ T cells, glucocorticoids, and cytokines converge to give a unified physiological response. In this study, we demonstrate that human Vγ2Vδ2 T cells augment MIF secretion. Remarkably, these Vγ2Vδ2 T cells, functioning similarly to MIF in part, counteracted inhibition of dexamethasone on production of IL-1β and TNF-α. SCID mice reconstituted with human PBMC that were mock depleted of Vδ2 T cells and repeatedly infected with lethal dose of Escherichia coli had shorter survival time than those reconstituted with PBMC that were depleted of Vδ2 T cells. Thus, human Vγ2Vδ2 T cells are likely to play broad-spectrum roles in immunoregulation and immunopathology by influencing MIF secretion and the immunomodulatory function of glucocorticoids.