Human T helper cells specific for antigens of typhus group rickettsiae enhance natural killer cell activity in vitro

Contribution of classical complement activation and IgM to the control of Rickettsia infection

Pathogenic Rickettsia are obligate intracellular bacteria and the etiologic agents of many life‐threatening infectious diseases. Due to the serious nature of these infections, it is imperative to both identify the responsive immune sensory pathways and understand the associated immune mechanisms that restrict Rickettsia proliferation. Previous studies have demonstrated that the mammalian complement system is both activated during Rickettsia infection and contributes to the immune response to infection. To further define this component of the mammalian anti‐Rickettsia immune response, we sought to identify the mechanism(s) of complement activation during Rickettsia infection. We have employed a series of in vitro and in vivo models of infection to investigate the role of the classical complement activation pathway during Rickettsia infection. Depletion or elimination of complement activity demonstrates that both C1q and pre‐existing IgM contribute to complement activation; thus implicating the classical complement system in Rickettsia‐mediated complement activation. Elimination of the classical complement pathway from mice increases susceptibility to R. australis infection with both increased bacterial loads in multiple tissues and decreased immune activation markers. This study highlights the role of the classical complement pathway in immunity against Rickettsia and implicates resident Rickettsia‐responsive IgM in the response to infection.

Rickettsiae stimulate dendritic cells through toll-like receptor 4, leading to enhanced NK cell activation in vivo

Adoptive transfer of Toll-like receptor (TLR) 4-stimulated dendritic cells (DCs) induces protective immunity against an ordinarily lethal rickettsial challenge, but the mechanism underlying this protection remains elusive. Therefore, we sought to determine the importance of TLR4 in early immunity to rickettsiae in vivo, particularly that conferred by TLR4-stimulated DCs. Rickettsial growth proceeded logarithmically in mice lacking TLR4 function, whereas in TLR4-competent mice rickettsial growth manifested a lag phase early, suggesting that TLR4 may initiate innate rickettsial immunity. TLR4-competent mice produced significant amounts of interferon (IFN)-gamma on day 1 of Rickettsia conorii infection, which was associated with significant expansion of the population of activated NK cells. Moreover, NK cells from TLR4-competent mice produced significantly higher levels of IFN-gamma and had greater cytotoxic activity than did those from TLR4-deficient mice. Last, adoptive transfer of rickettsiae-exposed, TLR4-stimulated DCs activated NK cells in vivo. Together, these data reveal an important role for DCs in recognizing rickettsiae through TLR4 and inducing early antirickettsial immunity.

The importance of the crystalline surface layer protein antigens of rickettsiae in T-cell immunity

Studies in animal models have demonstrated that solid immunity to typhus rickettsiae is dependent on immune T cells. In addition, the surface protein antigen (SPA) of typhus rickettsiae has been shown to be an effective immunogen, protecting vaccinated animals against subsequent challenge with virulent organisms. In the present studies we describe three classes of human lymphocytes which are capable of lysing cells infected with typhus rickettsiae. The first class is CD3,8-positive and is capable of specially lysing both HLA-matched and mismatched targets infected with typhus rickettsiae. Since this cytotoxic effector can be generated with IL-2 as well as with SPA it appears to be a lymphokine-activated killer (LAK). The second class of lymphocytes is CD3,4-positive and is capable of producing gamma interferon in response to the SPA of typhus rickettsiae. Gamma interferon in turn can cause the lysis of cells infected with typhus rickettsiae as well as inhibit intracellular rickettsial growth. A third cytotoxic effector which is CD3,4-positive and which is capable of lysing only HLA-matched targets infected with typhus rickettsiae was generated with a sonicated antigen, and its fine antigenic specificity is not known at present. We conclude that the SPA is an immunologically important protein for the human host and represents an outstanding candidate for a subunit vaccine against typhus.