Humoral immunity against the M. tuberculosis kasB persistent mutant

Abstract Mycobacterium tuberculosis (Mtb) is remarkably adept to establishing in an infected host a clinically silent latent state that can subsequently reactivates, posing a formidable hindrance to tuberculosis (TB) control. It is generally thought that the majority of the one-third of the world's population estimated to be infected with Mtb harbor latent bacilli. This latter population affords a large reservoir for the perpetuation of Mtb. The latent persistent form of Mtb is often drug-tolerant and thus difficult to treat. Given the propensity of Mtb to enter dormancy, it is likely that the infectious inoculum inhaled by a susceptible host contains latent form of bacilli in addition to those that are actively replicating. Ample evidence support the notion that Mtb, when exposed to conditions conducive to the establishment of a latent state, displays a gene expression profile distinct from that of actively replicating bacilli. Thus, it is likely that the antigenic profile of a latent persister is different than that of its rapidly growing counterpart. Indeed, loss of acid fastness has been demonstrated in Mtb existing in a chronic persistent state, which is thought to be, at least in part, the result of an aberrant cell envelope structure in dormant bacilli. Based on the above, it stands to reason that an effective TB vaccine should elicit an immune response that can target both actively replicating and latent persistent bacilli. In fact, BCG, the only anti-TB vaccine currently available and generated in conditions unrelated to those conducive to promote persistence (and therefore likely not expressing antigens (Ags) unique to latent tubercle bacilli), may not be capable of inducing an immune response that optimally protects against persistent organisms. The Jacobs group has recently characterized a set of mutants of Mtb kasB, which encodes a ?-ketoacyl-acyl carrier protein synthase involved in the biosynthetic pathway of mycolic acids (a major family of mycobacterial cell envelope lipids). The study has revealed that KasB activity is regulated via phosphorylation at two threonine residues, and that specific KasB-deficient mutants display phenotypes consistent with features of persisters, including a loss of acid fastness and inability to replicate when inoculated into mice. Importantly, immunization protocol that includes an acid-fast negative kasB persistent mutant -- which phosphorylation sites at the two aforementioned threonine residues have each been replaced by an asaparte (kasB-DD) -- protects against Mtb better than regimens that use BCG alone. The acid-fast negative kasB-DD strain displays an aberrant lipid profile involving species beyond mycolic acids. Aberrant packing of the cell envelope due to abnormal surface lipids such as mycolic acids may expose other macromolecules, including proteins and carbohydraes, that are otherwise masked. Collectively, these observations support the notion that persistent Mtb can express distinct Ags and that targeting such moieties, in addition to those typically present in actively replicating bacilli, might lead to enhanced vaccine efficacy. We therefore hypothesize that targeting Ags differentially expressed by and/or distinct to Mtb persisters represents an effective approach to developing anti- TB strategies including immunotherapeutics and vaccines. To begin testing this hypothesis, we propose to characterize the humoral immune response elicited by the acid-fast negative kasB-DD persister. The choice of this approach is based on emerging evidence suggesting antibodies (Abs) play a significant role in protection against Mtb and in modulating infection outcome. In addition, rigorous characterization of the Ab response to kasB-DD (and in the process, that of the control WT Mtb) can be expected to generate an extensive set of Mtb Ag-specific monoclonal Abs (mAbs) that constitutes a most valuable set of tools for advancing our understanding of three important aspects of Mtb research: (i) the humoral response of WT and persister tubercle bacilli, (ii) the biology of the difficult-to-track persistent Mtb, and (iii) the mechanisms that regulate tuberculous latency. Thus, the information yielded by these studies have the potential to lead to the development of novel strategies for better control of Mtb, including efficacious vaccines.