Bordetella pertussis and human monocyte-derived dendritic cells: a tool to identify an evading mechanism

Bordetella pertussis is the etiologic agent of pertussis or wooping cough , an highly, contagious and acute respiratory infection. B. pertussis is a strict human pathogen and transmission of disease occurs via respiratory droplets. B. pertussis posses different virulence factors classified in two categories: adhesins and toxins . Virulence factors facilitate the bacteria' s ability to adhere to respiratory cells and produce local and sistemic alterations in cell pathophysiology which contribute to microrganism replication and its capacity to evade host immune response. Adenylate cyclase toxin (ACT) is the B. pertussis factors which induces cell intoxication via cyclic AMP (cAMP) leading to inhibition of several immune functions allowing persistence of B. pertussis in the host and progression of infection. The mechanism underlying protection to B. pertussis infection is still a matter of debate. A central role in immunity is played by dendritic cells (DCs). DCs represent the initiator and modulator of the immune response that link innate and acquired immunity. DCs act as sentinels able to capture microbial antigens at the site of infection, this encounter drives the maturation process, a complex cascade of intracellular pathways that allows regulatory cytokine production, such as interleukin(IL-)12 p70 or IL- 10. After, DCs migrates to lymphonodes where they present antigens to naïve T cells and polarize the immune response. In this contest, DCs represent a tool to identify immune mechanisms which underly protection after B. pertussis infection. In the first part of the study we aimed to clarify the interaction of B. pertussis and human monocytederived DCs (MDDCs). we evaluated the ability of human MDDCs to phagocytose B. pertussis and the capacity of bacterial cells to survive intracellularly. A key point in the present study was the assessment of the ability of B. pertussis to influence MDDCs functions. In particular, we aimed to evaluate whether infected MDDCs undergo phenotypic and functional maturation, and which type of Th cells polarization is induced by infected MDDCs. The results shown that B. pertussis, while possessing a low susceptibility to be internalized by - and to survive in - MDDCs, triggers the onset of the maturation program and modulates cytokine production and antigen presenting cell functions. In particular by ELISA tests, we found that B. pertussis infected MDDCs are unable to induce the production of IL-12 p70 nevertheless driving Th1 immunity. DCs in response to pathogens can secrete not only IL-12 p70 but also a new described Th1 cytokine IL-23. IL-12 p70 and IL-23 are composed of two heterologous chains differently regulated, respectively p40/p35 and p40/p19. Quantitative RT-PCR has revealed that B. pertussis induces in MDDCs p40 and p19 (IL-23), but not p35 expression. MDDCs B. pertussis-infected drive a Th1 polarization and IL-23 may indirectly contribute to this polarization. Since ACT, as described in several studies, inhibits lipopolysaccaride (LPS)-driven IL-12 p70 production in human, to evaluate if ACT was involved in the unability of B. pertussis to induce the production of IL- 12 p70 we infected MDDCs with a wild-type strain (BpWT) and its isogenic ACT-deficient mutant (BpACT¯) in presence or absence of a cAMP analogus molecule d-butyrril-cAMP (d-cAMP). Indeed BpACT¯infection induced consistent production of IL-12 p70 and IL-12 p35 transcription. Addition of the d-cAMP abolished IL-12 p70 production and IL-12 p35 expression in BpACT¯-infected MDDCs. BpACT¯ infection induced the expression of the transcription factors interferon regulatory factor 1 (IRF- 1) and IRF-8 and of beta interferon (IFN-β), involved in IL-12 p35 regulation, and the expression of these genes was inhibited by d-cAMP addition and in BpWT-infected MDDCs. The concomitant expression of IL-12 p70 and IL-23 allowed BpACT¯ to trigger a more pronounced T helper 1 polarization compared to BpWT. Thus in conclusion the present study demonstrate that ACT-dependent cAMP induction leads to the inhibition of pathways ultimately leading to IL-12 p35 production, thus representing a mechanism for B. pertussis to escape the host immune response. Further studies could be important to recognize virulence factors or immune reactions pivotal in B. pertussis infection useful to enhance the protective capacity of the acellular pertussis vaccines.