Active Evasion of CTL Mediated Killing and Low Quality Responding CD8+ T Cells Contribute to Persistence of Brucellosis

Abstract

Brucellosis is a common zoonotic disease that remains endemic in many parts of the world. Dissecting the host immune response during this disease provides insight as to why brucellosis is often difficult to resolve. We used a Brucella epitope specific in vivo killing assay to investigate the ability of CD8+ T cells to kill targets treated with purified pathogenic protein. Importantly, we found the pathogenic protein TcpB to be a novel effector of adaptive immune evasion by inhibiting CD8+ T cell killing of Brucella epitope specific target cells in mice. Further, BALB/c mice show active Brucella melitensis infection beyond one year, many with previously unreported focal infection of the urogenital area. A fraction of CD8+ T cells show a CD8+ Tmem phenotype of LFA-1hi, CD127hi, KLRG-1lo during the course of chronic brucellosis, while the CD8+ T cell pool as a whole had a very weak polyfunctional cytokine response with diminished co-expression of IFN-γ with TNFα and/or IL-2, a hallmark of exhaustion. When investigating the expression of these 3 cytokines individually, we observed significant IFN-γ expression at 90 and 180 days post-infection. TNFα expression did not significantly exceed or fall below background levels at any time. IL-2 expression did not significantly exceeded background, but, interestingly, did fall significantly below that of uninfected mice at 180 days post-infection. Brucella melitensis evades and blunts adaptive immunity during acute infection and our findings provide potential mechanisms for the deficit observed in responding CD8+ T cells during chronic brucellosis.

Humoral Immunity and CD4+ Th1 Cells Are Both Necessary for a Fully Protective Immune Response upon Secondary Infection with Brucella melitensis

“…A recent report suggests that the failure of the immune system to maintain a CD8 + T cell response during chronic brucellosis results from bacterial evasion dependent on the virulence factor TcpB (65). The identification of CD4 + T cells as key lymphocyte subsets is critical to determine which In this study, we were unable to confer sterilizing protection in the spleen of naive mice by the transfer of serum or peritoneal cells from the live-immunized group (data not shown).…”

“…The late immune protection was scored by the frequency of animals that were not able to completely eradicate bacteria from their spleen at 50 d postchallenge. The spleen was chosen as the control organ because Brucella has been shown to persist for long periods of time (up to 100 d) in this organ (65). In our model, the absence of bacteria in the spleen has been always correlated with complete elimination of bacteria in the liver (data not shown).…”

Identification of Immune Effectors Essential to the Control of Primary and Secondary Intranasal Infection with Brucella melitensis in Mice

“…Brucella challenge. Thus, although Brucella display immune escape mechanisms that affect CD8 + T cell activity (73,74), memory CD8 + T cells seem to be able to compensate for the absence of memory CD4 + T cells and, thus, to control secondary i.n. infection with Brucella.…”

CD8 + T Cell Exhaustion, Suppressed Gamma Interferon Production, and Delayed Memory Response Induced by Chronic Brucella melitensis Infection

“…Brucella has many mechanisms to survive and replicate in hostile host cells, including inducing the unfolded-protein response (UPR), hijacking host nutrients, and counteracting the effects of pH changes, among many others (3)(4)(5)(6). The chronic, reactivating nature of Brucella infection, along with its stealthy intracellular life-style, makes infections difficult to clear and requires lengthy antibiotic treatment (7)(8)(9). CD8 ϩ T cells control intracellular infections by identifying and killing compromised host cells as a part of the adaptive immune response (10,11).…”