How do macrophages kill TB? How does TB evade macrophage defenses?
Macrophages are the central player in TB pathogenesis as they serve both as the host cell for infection and the cell primarily responsible for killing bacteria. Early after infection TB replicates extensively in macrophages, however when T cells arrive in the infected lungs, they activate microbicidal mechanisms of macrophages. One major mechanism by which T cells activate macrophages to control infection is by production of the cytokine IFN-γ. Surprisingly, we lack a complete understanding of the molecular mechanisms of macrophages that are activated by IFN-γ that result in direct killing of bacteria. One major focus of the lab is to identify new effector mechanisms activated by IFN-γ that facilitate successful control of infection using both hypothesis driven approaches and unbiased screening. In newer projects, we are also examining IFN-γ independent mechanisms by which T cells activate macrophages to control infection. Finally, although activation of macrophages can result in immune control, it rarely results in sterilization. We also investigate the mechanisms by which bacteria are able to prevent clearance by these immune mechanisms.
Host-derived aldehydes as antibacterial effectors
The production by macrophages of reactive oxygen species (ROS) by the phagosome oxidase complex and nitric oxides (NO) by inducible NO synthase has long been appreciated and characterized. However, there exist several reactive aldehydes that are produced as a consequence of increased metabolism by infected aldehydes (such as formaldehyde, FA) or lipid peroxidation in phagosomes (such as 4-hydroxynonenal, 4-HNE, and malondialdehyde, MDA), all of which are highly toxic to bacteria. Findings from the lab and other groups have also shown that methylglyoxal, MG, can spontaneously form from DHAP and has bactericidal activity (many bacteria have evolved MG detoxification systems to combat this). We are particularly interested in the fact that a deficient allele of Aldh2 (aldehyde dehydrogenase 2), is extremely common (present in ~8% of the human population and 35-40% of East Asians). Deficiency in this gene is classically associated with Asian flush syndrome and deficiencies in alcohol metabolism, yet this supposedly-deleterious allele remains highly frequent among the human population. We posit that deficiency in aldehyde detoxification yields an evolutionary advantage to the host, allowing for the accumulation of these reactive toxic intermediates to combat bacteria during infection.
Metabolic regulation of the host-pathogen interaction
During infection, Mtb is thought to exist along a spectrum of metabolic states, from active replication through a dormant non-replicating state. Mtb metabolism has a significant impact on the efficacy of antibiotics, as slowly replicating or dormant bacteria are very difficult to kill with conventional antibiotics. Macrophage metabolism is also important for the outcome of infection, as shifts in macrophage metabolism affect not only microbicidal mechanisms such as autophagy, but also affect the spectrum of nutrients available to the bacterium. In vivo, Mtb is thought to rely on host lipids and cholesterol as primary carbon sources during infection. We are interested in understanding the intersection between macrophage metabolism and bacterial metabolism, and use a variety of techniques ranging from bacterial and host genetics through metabolomics and proteomics to address these questions. An interesting observation through genome-wide genetic screens of Mtb is a conserved D- and L-lactate utilization system that has previously been unannotated and under-appreciated.
Development of novel therapeutics and vaccine adjuvants
Our research is motivated by the need to find new interventions for treating and preventing infection with Mtb to support efforts for global eradication. One major area of emphasis is the discovery of host-targeted therapeutics that function to enhance the immune response to Mtb or inhibit virulence pathways to facilitate clearance of infection. We use basic research to identify important pathways in the host that can be targeted by small molecules.
