Although the production of interferon-gamma by CD4+ T lymphocytes may perform a central role in the development of the immunity against Mycobacterium tuberculosis,1 a subpopulation of T lymphocytes called Gamma-Delta (TCR-γδ) also plays a significant role, not yet completely defined, in mycobacterial infections. Empirical data support that these cells promote important effector functions such as proliferation, release of T helper type 1 cytokines and cytotoxic activity.2

TCR-γδ may acquire the ability to process and to present antigens. Experimentally, TCR-γδ are capable of expression of the antigen presenting phenotype characterized by the HLA-DR, CD80 and CD86 markers. The magnitude of this expression is comparable to that observed in dendritic cells, considered professional APCs (antigen presenting cells) standards.3 Although there are results suggesting a specific role for TCR-γδ in the activation of αβ T cells, the relevance of TCR-γδ capacity to act as APCs has yet to be confirmed in vivo.3,4

In tuberculosis, human TCR-γδ can produce different functional responses. Cells stimulated by Bacillus–Calmette–Guérin (BCG) or mycobacterial lysates can produce cytotoxic granules (granulisins) and other antimicrobial peptides, and show cytolytic activity after co-culture with human monocytes infected with mycobacteria.5

Additionally, the production of granules by cytotoxic T lymphocytes (CD8+) and TCR-γδ has been characterized as an important adjuvant mechanism. Proteins such as perforin, granzyme B and granulisin have the property of lysing cells infected by M. tuberculosis, or even directly lysing the bacilli.6

The coinfection of Human Immunodeficiency Virus (HIV) and tuberculosis results in the enhancement of the activity of both diseases.7 HIV infection affects several mechanisms of the immune response, including T CD4, dendritic cells and TCR-γδ, contributing to the loss of immune competence. HIV infection may directly impact the repertoire and effector function of TCR-γδ.8 Moreover, there is a loss of phosphoantingen recognition ability of TCR-γδ, which could affect their function as APCs,9,10 and reduction of cytotoxic granules production by T CD8 cells, which could be recovered with antiretroviral therapy.11

About 20 years ago, with the introduction of combined antiretroviral therapy (cART), control of HIV infection allowed a highly lethal disease to be seen as a chronic condition. HIV-infected individuals currently have encouraging prospects for good survival and quality of life.12 Young people infected by vertical transmission present infection activity from the beginning of life. cART has significant efficacy in this population, but the preservation of immune function may be incomplete.13 Based on this scenario, our study aimed to evaluate the antigen-presenting and activation phenotypes and the capacity to produce cytotoxic granules in response to mycobacterial antigens by CD8+ and TCR-γδ in a sample of young people infected with HIV by vertical transmission, vaccinated with BCG in childhood.

Our study showed that the overall ability to produce cytotoxic granules by CD8+ and TCR-γδ cells of vertically HIV-infected youth, under effective therapy, is comparable to that of healthy individuals in response to mycobacterial antigens, and lower, in TCR-γδ cells, in response to a mitogen. There are few reports in the literature on the production of lymphocyte granules in immunity to mycobacteria in HIV-infected patients. A study in adults coinfected with HIV and latent tuberculosis showed that CD107a expression by CD8 + T-lymphocytes was lower in HIV-infected patients than in seronegative individuals.16 In different settings, several reports have shown that production of granules after stimulation with peptides or HIV capsid proteins was reduced with the progression of infection in adults and children.17,18 In that regard, a report from a pediatric cohort demonstrated greater preservation of granule production in slow progressors.19 The production of granules for other antigens, such as those derived from Varicella-Zoster virus, has been shown to be lower in infected children than in healthy controls.20 However, the production of granules for viral capsid proteins may recover in cases of effective therapy.21,22 In relation to phosphoantigen capacity of stimulating the production of granules by TCR-γδ lymphocytes, a direct relation with the preservation of the number of CD4+ T lymphocytes was demonstrated.23 Evaluated in conjunction with the literature, our findings suggest that the production of granules can be preserved in HIV-infected individuals on effective therapy. The preservation of this important defense mechanism against mycobacteria may be relevant in the context of a chronic and lifelong infection.

Significantly lower expression of CD107a by TCR-γδ lymphocytes, in response to mitogen, observed in infected youths, may suggest that the immune recovery afforded by antiretroviral therapy is not complete. In an analogous scenario, reduced functionality and amount of TCR-γδ lymphocytes was observed in adults with chronic HIV infection.24

The higher expression of CD107a by CD8+ T-lymphocytes in the group of patients in immunological class 3 may suggest the therapeutic efficacy of cART, with immune restoration. An analogous scenario was described in reports evaluating the expression of CD107a by CD8+ and TCR-γδ cells stimulated with viral capsid proteins, with an increase in this expression associated with effective cART.21,22

In the group of HIV-infected patients, we observed increased expression of HLA-DR by non-stimulated γδ T-cells and by CD8 + T cells cultured with MTB. These results, in our view, reflect a state of inflammatory activation secondary to chronic disease, even adequately treated, in the HIV group. The importance of persistent inflammation and activation has been highlighted as one of the causes of sustained morbidity in cohorts of infected individuals with successful treatment.25

In our study, the expression of antigen presenting cell phenotype (HLA-DR+, CD80+ and CD86+) after stimulation with mycobacterial antigens, in both HIV-infected and control groups, was identified in the TCR-γδ subpopulation. This phenotype was described in professional APCs.3 Experimental data demonstrating that this subpopulation of lymphocytes would be able to perform this function has been reported.26 We did not find in the literature any study identifying this function in patients infected by HIV after stimulation with mycobacterial antigens. The relevance of this function is potentially high, considering the epidemiological importance of HIV-tuberculosis coinfection.

In conclusion, our results suggest that vertically HIV-infected youth undergoing effective treatment have preserved the production of granules by T-lymphocytes in response to mycobacterial antigens. Activation marker expression, either spontaneous or stimulated by mycobacterial antigen, was increased in the HIV-infected youth. These results underscore the importance of the careful monitoring of this population, in order to evaluate their ability to respond to mycobacteria and the inflammatory state throughout life.

This work was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (São Paulo Research Foundation) Grant number 2013-26862/9, and CAPES Foundation, Federal Government of Brazil — M.P.A. Master of Science Scholarship.

The authors declare no conflicts of interest.