DC-MTB INTERACTIONS · Visualization of Dendritic cell IL-12 production and engagement with antigen-specific T cells during Mycobacterium tuberculosis infection in vivo

Visualization of Dendritic cell IL-12 production and engagement with antigen-specific T cells during Mycobacterium tuberculosis infection in vivo

Every second a new person becomes infected with the intracellular bacterial pathogen, Mycobacterium tuberculosis (Mtb). Mtb is the etiological agent of tuberculosis, the leading cause of death in the World due to a single microorganism. The live, attenuated strain of M. bovis called Bacille Calmette-Guérin (BCG) is the only available vaccine against Mtb infection. It lacks clinical efficacy but is safe as a vaccine. Improving BCG is therefore considered a valid strategy towards global control of tuberculosis. This can only be achieved on a rational basis by understanding how mycobacteria and in particular BCG, prime CD4+ T cells in vivo. T cell priming is the process in which T cells of the adaptive immune system become activated and armed to combat specific pathogens. This complex process unfolds mainly in lymph nodes (LNs), structures distributed throughout the body that drain fluid from distal tissue sites via lymphatic vessels. LNs trap circulating T cells to facilitate chance encounters with Dendritic cells (DCs), stellate-shaped phagocytes of the innate immune system. DCs have superior skills in antigen presentation, co-stimulation and cytokine production. DCs provide these signals to naïve T cells at the necessary levels needed to activate, arm and expand T cells into an effector population. Through the production of interleukin (IL)-12, DCs differentiate naïve CD4+ T cells during priming into an effector population armed to produce the cytokine Interferon (IFN)-gamma, called Th1 cells, which are central for fighting-off Mtb infection. Although BCG has been in use for almost 100 years, there is a paucity of information on immune-cell interactions that unfold in the LN that drains the BCG vaccination site in the skin. In fact, the intradermal delivery of BCG could in part explain its lack of efficacy. BCG does not gain direct access to lymphatic vessels so it falls upon skin DCs at the site of BCG inoculation to internalize and transport BCG into the draining LN. This channeling of BCG into the draining LN and the formal engagement of DCs with naïve CD4+ T cells therein both remain poorly studied but are essential for T-cell priming. It is thus unclear if BCG is presented to naïve CD4+ T cells in the LN by migratory skin DCs or by LN-resident DCs that acquire BCG or its metabolites from migratory skin DCs. This is further complicated by the fact that there are several DC sub-populations in both the skin and LN, most of which remain functionally ill-defined, especially with regards to their involvement in the triggering of a Th1 response to mycobacteria. We have set up a mouse model to begin addressing some of these caveats. Our studies show that the population of CD103- CD326- double-negative dermal DCs (DN dDCs) are the main migratory skin DC subset entering the LN that drains the BCG vaccination site in the skin. In contrast to earlier studies showing a role for granulocytes, the migratory skin DC population is the main cell type moving from the skin into the draining in our model. Importantly, the influx of DN dDCs into the BCG-draining LN parallels the onset of CD4+ T-cell priming at this site. We also show that DN dDCs transport BCG into the draining LN in vivo via a pertussis toxin-sensitive mechanism. In addition, Caspase-1 and MyD88, two intracellular signaling molecules central for DC activation were found to be partially required for DC entry and BCG transport into the LN. The cytokines TNF and IL-12p40, which are downstream of Caspase-1 and MyD88 signaling, were also partially required for entry of DCs and BCG into the draining LN. Using a DC adoptive transfer approach we found that wild-type DCs were unable to migrate from skin into draining LN when transferred into Caspase-1- or MyD88-deficient hosts, suggesting that these factors do not necessarily regulate DC migration into LNs at the level of the migrating DC itself. Interestingly, infiltrating skin DCs homed exclusively to the LN paracortex, the T-cell area of the LN, where they were found in close apposition to BCG-specific CD4+ T cells, suggesting a possible, direct role for these DCs in priming T cells in vivo. In all, our observations establish DN dDCs as an important population in the priming of CD4+ T cells in the BCG-draining LN and identify key inflammatory molecules that regulate this process in vivo. A better understanding of how DCs communicate with CD4+ T cells in mycobacteria-infected LNs yields valuable insights on immune-cell interactions occurring at the interface of innate and adaptive immunity and provide in particular, new perspectives for the manipulation of DCs for clinical benefit during vaccination against tuberculosis.

Data: CORDIS, © European Union