In this study, we have demonstrated that DC-mediated HIV trans-infection is increased significantly following exposure to M. tuberculosis. Enhancement was observed both in in vitro-cultured MDDCs and in myDCs and was independent of the mycobacterial strain used. Additionally, increased trans-infection was observed using either T cell lines or primary autologous CD4 T cells. The response was dose and time dependent and was accompanied by upregulation of DC activation markers. Heat-killed M. tuberculosis had a similar stimulatory effect, suggesting that bacterial replication is not required for increased trans-infection of T cells by DCs. The heat-killed M. tuberculosis result also indicates that increased trans-infection is caused by a mycobacterial component interacting with the DC rather than by a molecule that is newly synthesized and secreted from the mycobacterium.
can induce changes in DC function via ManLAM/DC-SIGN interactions (15
); however, even in the absence of ManLAM in the bacterial cell wall, M. tuberculosis
can alter DC function (1
). Our results indicate that ManLAM/DC-SIGN interactions are not required to induce increases in trans
-infection. Exposing the DCs to ManLAM had little to no effect on surface activation marker expression or the ability to trans
-infect. DC-SIGN blocking antibodies were similarly ineffective. Blocking antibodies did not prevent M. tuberculosis
-induced upregulation of surface activation markers. While the blocking antibodies appeared to decrease HIV transfer, this observation is likely due to the blocking antibodies interfering with HIV binding to DC-SIGN, one of the main receptors involved in trans
-infection. Taken together, our results point toward a mycobacterial pathogen-associated molecular pattern (PAMP) interacting with a pattern recognition receptor (e.g., a Toll-like receptor [TLR]) expressed on the DCs to mediate increased trans
-infection. Consistent with this, others have shown that M. tuberculosis
is a potent TLR2 agonist that can also act to a lesser degree through TLR4 and -9 (3
infection is chronic, with a course of disease observed in months rather than days. Granulomatous lesions that harbor persistent M. tuberculosis
are thought to be highly dynamic structures, allowing constant leukocyte migration between the lung and lymphoid tissues (11
). Our data show that M. tuberculosis
increases the magnitude of HIV trans
-infection 2.2- to 7.0-fold. In general, between 1 and 10 CFU of mycobacterium per DC was required to enhance HIV trans
-infection. While these numbers appear high, the microenvironment of M. tuberculosis
infection varies greatly and likely contains regions where these levels are physiologically relevant. Cavitary TB, for example, carries an extremely high burden of mycobacterium, in which the multiplicity of infection (MOI) is probably even higher than 10:1. Even the relatively modest effects observed with lower doses of M. tuberculosis
could, over the long term, easily result in significant changes to the final outcome of TB. HIV-negative individuals have a 5 to 10% lifetime risk of developing TB, whereas in coinfected individuals this number jumps to a 5 to 10% risk per year (23
). Our data suggest that a loss of antigen presentation combined with an increased HIV trans
-infection could contribute to the loss of immune control of M. tuberculosis
in the setting of HIV infection.
We further showed that M. tuberculosis
-exposed DCs concentrated HIV into plasma membrane-derived, pocketlike structures that remained accessible to surface-applied antibodies, indicating that the sequestered HIV was not endocytosed and instead remained intact and competent to trans
-infect CD4 T cells. These data extend our previous analysis of LPS-stimulated HIV sequestration into an identical structure (42
) and suggest that pocket formation is a generalized response to activating stimuli. Indeed, Candida albicans
exposure leads to partial activation of DCs along with increased HIV trans
), suggesting that innate immune responses to invading copathogens can act to stimulate HIV dissemination.
Dendritic cells remain a key cell type in which to study coinfection. M. tuberculosis
infects both macrophages and DCs in the lungs. While macrophages remain primarily in the lung tissues, DCs migrate to the lymph nodes after phagocytosis of M. tuberculosis
in order to initiate immune responses (39
). Our imaging analysis showed that M. tuberculosis
exposure induced HIV sequestration even in DCs that did not harbor M. tuberculosis
(Fig. , , and ). These data support the hypothesis that microbial products or inflammatory cytokines released by the infected DCs can affect neighboring DCs and increase their ability to trans
-infect, even though they may not harbor intact M. tuberculosis
. This suggests that M. tuberculosis
-exposed DCs trafficking from the lung could further exacerbate HIV disease by activating trans
-infection in lymphoid tissue-resident DC populations.
Finally, we found that M. tuberculosis
exposure dramatically reduced processing and presentation of HIV virions, whereas presentation of exogenously added peptides was unimpaired, indicating that the DCs retained the ability to present processed antigens but could not degrade and present newly acquired virions. Our data suggest that endocytic downregulation is accompanied by HIV sequestration off the cell surface into pocketlike membrane invaginations, leaving the virus intact and capable of infecting CD4 T cells at the infectious synapse rather than trafficking into the lysosomal degradation and antigen presentation pathway. This result is supported by the findings of Henderson et al., who demonstrated a downmodulation of endocytosis in DCs after M. tuberculosis
We propose that one way in which M. tuberculosis infection exacerbates HIV disease is by activation of DC trans-infection and concurrent reduction of de novo antigen processing and presentation. Myeloid DCs that encounter M. tuberculosis in the lungs become activated and migrate to locally draining lymph nodes, where they can induce M. tuberculosis-specific CD4 T cell responses. In the context of HIV infection, M. tuberculosis-induced defects in DC function act both to suppress specific immune control of HIV and to enhance DC-mediated HIV trans-infection of CD4 T cells. A single DC can interact with dozens of T cells in a lymph node in a day, providing ample opportunity to acquire and disseminate HIV among the lymphoid T cells. Enhanced HIV spread, therefore, is predicted to target primarily the CD4 T cell population, further hastening the decline of this cell population and the collapse of immune control of both pathogens.