We generated mice tg for a TCR specific to the dominant M. tuberculosis antigen ESAT-6 to investigate the ability of Th1 cells to provide protective immunity. Our analysis of adoptively transferred C7 TCR tg CD4+ T cells revealed that priming of naive ESAT-6–specific TCR tg cells occurred 7–10 d after aerosol infection with a low dose of M. tuberculosis. Our experiments demonstrated that adoptively transferred Th1 cells specific for ESAT-6 can provide a high level of protection, as measured up to 90 d after infection, but that in vivo mycobacterial growth is unimpaired for the first week of infection despite large numbers of pathogen-specific T cells in the lungs.
The delay in priming of naive ESAT-6–specific T cells is not unexpected and is consistent with the finding that M. tuberculosis
–specific effector T cells are not present at high frequencies in the lungs of mice until ~3 wk after infection (24
). Our results are also consistent with a recent study (34
) that monitored the priming of TCR tg CD4+
T cells specific to the M. tuberculosis
antigen Ag85B. One notable difference is that priming of Ag85B-specific monoclonal CD4+
T cells occurred at day 12 after aerosol infection, 2–4 d later than the C7 TCR tg cell response documented in this study. Differences in priming between naive Ag85B and ESAT-6–specific clones may result from differences in the expression patterns of these two proteins (32
), differences in the presentation of these antigens, or differences in the sensitivity of the TCRs used by Ag85B TCR tg cells and C7 TCR tg cells for their cognate antigens. Direct comparative studies will be required to distinguish between these possibilities.
Our studies with in vitro–generated Th1 cells revealed two surprising findings. One is how well monoclonal ESAT-6–specific Th1 cells were able to protect mice from M. tuberculosis infection. The second is the fact that despite high numbers of C7 Th1 effector cells in animals before infection, bacterial replication was not controlled until 1 wk after infection.
The 2-log reduction in the number of bacilli in animals that received C7 Th1 cells is greater than that observed after BCG immunization and comparable to prime-boost vaccination methods (35
). By using adoptively transferred C7 Th1 cells, our experiments demonstrate that monoclonal T cells specific to ESAT-6 can provide a level of early protection that is comparable to that provided by mixed populations of M. tuberculosis
effector T cells after vaccination. Thus, complex populations of T cells are not required to control infection, a result that suggests that adoptive T cell transfer may provide a therapeutic option for drug-resistant M. tuberculosis
It is presumed that improved vaccines can be designed to protect individuals from M. tuberculosis
infection by eliciting robust antigen-specific T cell responses. This view is supported by the observation that after aerosol infection in mice, accumulation of M. tuberculosis
T cells in the lungs coincides with the control of bacterial growth (6
). Several hypotheses for a lack of sterilizing immunity to M. tuberculosis
can be proposed, including a delayed recruitment of effector T cells to the lungs. Our ability to track adoptively transferred M. tuberculosis
–specific C7 Th1 cells allowed us to determine that approximately a half million of these cells were in the lungs before infection and allowed us to definitively rule out this hypothesis.
What accounts for the delayed protection by C7 Th1 cells? Given the low dose infection used in our studies (100 bacteria) and the slow doubling time of M. tuberculosis
during the first several days of infection, very few cells may be infected, and the probability of C7 Th1 cells “finding” an infected cell may be low. Although several cell types, including macrophages and DCs, are infected by day 14 after infection (38
), no information exists on the frequency or phenotype of infected cells in the lungs during the first week of infection (although presumably alveolar macrophages are the first cells infected). A recent study using multiphoton microscopy found that liver-resident macrophages, Kupffer cells, were the first cells infected with BCG after i.v. infection (39
). Recruitment of uninfected Kupffer cells and blood-derived monocytes to infected cells occurred 2 wk after infection.
If similar delays in cell recruitment occur in the lungs after M. tuberculosis infection, it could take 7–10 d until lung resident cells (i.e., macrophages, DCs, and effector/memory T cells) home to infected cells. Before day 7 after infection, C7 Th1 cells may randomly encounter infected cells, whereas after day 7, migration within the lungs to infected sites could be directed by inflammatory signals. Our studies with naive C7 CD4+ T cells demonstrated that T cell priming in pLNs also occurred only after day 7 following infection. A similar mechanism may account for the delayed priming of naive CD4+ T cells. After the first week of infection, DCs and other APCs may home to infected sites within the lung and traffic to draining LNs to prime naive T cells. This hypothesis requires further investigation.
On the other hand, if C7 Th1 cells home to infected cells within the lungs promptly after infection, it would suggest defective collaboration between CD4+
effector cells and infected cells: infected cells may be unable to present antigen to activate effector CD4+
T cells, or infected cells may be unable to control bacterial replication after stimulation by IFN-γ and other factors provided by effector CD4+
T cells. In vitro studies support both hypotheses, but in vivo studies to answer these important questions are necessary (40
The findings reported in this paper have important implications for understanding immunity to M. tuberculosis. In this study, we show that high numbers of ESAT-6–specific Th1 cells provided substantial protection to mice after low dose aerosol infection with M. tuberculosis. However, this protection was delayed until after day 7 following infection, and bacteria persisted in these animals thereafter. This finding may help explain why vaccination against M. tuberculosis has been difficult: immediately after infection, M. tuberculosis resides in cells that CD4+ T cells cannot help.