Two recent papers suggest that limitations on the activation of TB-specific CD4 T cells may contribute to the persistence of the infection. In PLoS Pathogens, Tyler Bold and colleagues show that only a small percentage of CD4 T cells targeting TB’s Ag85B peptide 25 produce the cytokine interferon gamma in the lungs of infected mice. The percentage also declines (from ~10% to less than 1%) as the infection progresses from the acute to the chronic phase. In looking for an explanation, the researchers found the decline in CD4 T cell activation paralleled a decrease in the expression of the fbpB gene that encodes the Ag85B antigen. Infection of mice with a recombinant TB designed to maintain fbpB expression led to 2-5 fold higher levels of antigen-specific CD4 T cell activation and also impaired TB persistence (bacterial load in the lungs of these mice was ~10-fold lower).
In the journal Immunity, researchers from the laboratory of Ron Germain use imaging techniques to demonstrate impaired CD4 T cell activation in TB granulomas in the livers of infected mice. By staining TB-specific CD4 T cells with dyes and capturing their activities within granulomas on video, the study reveals that only a small fraction of the total number of cells stop to engage with antigen and produce cytokines. The authors note that: “these findings suggest that limited antigen presentation and/or recognition within granulomas evoke a muted T cell response drawing on only a fraction of the host's potential effector capacity.” Although access to full text requires a subscription, the quicktime videos can be accessed directly, links and captions are appended below.
Both teams of researchers suggest that strategies to increase TB-specific CD4 T cell activation by derepressing antigen expression may have immunotherapeutic potential. Ron Germain and colleagues also state: “An implication of these findings is that strategies aimed solely at expanding the pool of antigen-specific effector T cells in individuals infected with some persistent pathogens, such as Mtb, may meet with limited success, because there may be insufficient antigen present at sites of infection to support additional effector responses.” This could potentially explain the lack of efficacy seen in studies of Mycobacterium vaccae immunotherapy for the treatment of TB.
PLoS Pathog 7(5): e1002063. doi:10.1371/journal.ppat.1002063
Tyler D. Bold1, Niaz Banaei2¤a, Andrea J. Wolf2¤b, Joel D. Ernst1,2,3*
1 Department of Pathology, New York University School of Medicine, New York City, New York, United States of America, 2 Division of Infectious Diseases, Department of Medicine, New York University School of Medicine, New York City, New York, United States of America, 3 Department of Microbiology, New York University School of Medicine, New York City, New York, United States of America
Adaptive immunity to Mycobacterium tuberculosis controls progressive bacterial growth and disease but does not eradicate infection. Among CD4+ T cells in the lungs of M. tuberculosis-infected mice, we observed that few produced IFN-γ without ex vivo restimulation. Therefore, we hypothesized that one mechanism whereby M. tuberculosis avoids elimination is by limiting activation of CD4+ effector T cells at the site of infection in the lungs. To test this hypothesis, we adoptively transferred Th1-polarized CD4+ effector T cells specific for M. tuberculosis Ag85B peptide 25 (P25TCRTh1 cells), which trafficked to the lungs of infected mice and exhibited antigen-dependent IFN-γ production. During the early phase of infection, ~10% of P25TCRTh1 cells produced IFN-γ in vivo; this declined to <1% as infection progressed to chronic phase. Bacterial downregulation of fbpB (encoding Ag85B) contributed to the decrease in effector T cell activation in the lungs, as a strain of M. tuberculosis engineered to express fbpB in the chronic phase stimulated P25TCRTh1 effector cells at higher frequencies in vivo, and this resulted in CD4+ T cell-dependent reduction of lung bacterial burdens and prolonged survival of mice. Administration of synthetic peptide 25 alone also increased activation of endogenous antigen-specific effector cells and reduced the bacterial burden in the lungs without apparent host toxicity. These results indicate that CD4+ effector T cells are activated at suboptimal frequencies in tuberculosis, and that increasing effector T cell activation in the lungs by providing one or more epitope peptides may be a successful strategy for TB therapy.
Immunity, Volume 34, Issue 5, 807-819, 19 May 2011
Jackson G. Egen, Antonio Gigliotti Rothfuchs, Carl G. Feng, Marcus A. Horwitz, Alan Sher, Ronald N. GermainSee Affiliations
Effector T cells within mycobacterial granulomas show limited antigen activation
Effector T cells within granulomas have submaximal responses
Low frequency of arrested effector T cells display polarized cytokine secretion
Effector T cell motility and cytokine production have an inverse relationship
Cell-mediated adaptive immunity is critical for host defense, but little is known about T cell behavior during delivery of effector function. Here we investigate relationships among antigen presentation, T cell motility, and local production of effector cytokines by CD4+ T cells within hepatic granulomas triggered by Bacille Calmette-Guérin or Mycobacterium tuberculosis. At steady-state, only small fractions of mycobacteria-specific T cells showed antigen-induced migration arrest within granulomas, resulting in low-level, polarized secretion of cytokines. However, exogenous antigen elicited rapid arrest and robust cytokine production by the vast majority of effector T cells. These findings suggest that limited antigen presentation and/or recognition within granulomas evoke a muted T cell response drawing on only a fraction of the host's potential effector capacity. Our results provide new insights into the regulation of host-protective functions, especially how antigen availability influences T cell dynamics and, in turn, effector T cell function during chronic infection.
Dye-labeled, in vitro stimulated p25 and OT-II T cells were transferred i.v. into BCG-infected mice (2 weeks p.i.). Four hours later, animals were subjected to intravital hepatic imaging and 4-dimensional data series were collected. Shown are examples of T cell migration in four independent granulomas. Different dye combinations were used for different experiments and the colors representing OT-II and p25 T cells are indicated. Time shown in hr:min:s. Scale bars represent 30 μm. Related to Figure 2.
Dye-labeled, in vitro stimulated p25 and OT-II T cells were transferred i.v. into BCG-infected mice (2 weeks p.i.). Four hours later, animals were subjected to intravital hepatic imaging and 4-dimensional data series were collected. Two examples of Ova323 and Ag85b240 peptide-induced T cell arrest are shown sequentially. Peptides were injected i.v. at the indicated time points and colors representing OT-II and p25 T cells are shown for each image series. Time shown in hr:min:s. Scale bar represents 40 μm. Related to Figure 2.
Adoptive transfer and infection were performed as described in Figure 3A. At 2 weeks p.i., animals were subjected to intravital hepatic imaging, collecting 4-dimensional data series. EGFP-expressing cells are shown in green. Time shown in min:s. Scale bar represents 30 μm. Related to Figure 3.
Adoptive transfer and infection with rBCG30 were performed as described in Figure 3A. At 2 weeks p.i., a 3-dimensional image was captured from a fixed liver section. EGFP-expressing cells are shown in green, Ag85b staining in white, and IFN-γ staining in red. Related to Figure 5.
Four-dimensional data series from a LysM-EGFP mouse that had been infected with Mtb 2 weeks prior to imaging. Sinusoids were visualized by injecting BSA-647 i.v. immediately prior to imaging. EGFP-expressing cells are shown in green. Time shown in min:s. Scale bar represents 25 μm. Related to Figure 6.
CMFDA-labeled p25 (red) and CMTPX-labeled OT-II T (green) cells were transferred i.v. into Mtb-infected mice (2 weeks p.i.). Four hours later animals were subjected to intravital hepatic imaging, collecting 4-dimensional data series. Time shown in min:s. Scale bar represents 30 μm. Related to Figure 6.