Activation of NKT and NK cells is observed upon infection with a broad range of pathogens including bacteria, viruses, parasites and fungi. Cytokines, particularly IL-12 and IL-18 released from toll-like receptor (TLR) activated APCs during infection contribute to the activation of both cell populations (1
). However, while NKT cells recognize endogenous mammalian or exogenous bacterial glycosphingolipids (GSLs) presented by CD1d on APCs (3
) through their conserved semi-invariant mouse TCR, NK cell activation involves activating and inhibitory receptors that recognize “self”, “missing self” and “altered or stressed self” (4
Although NKT and NK cells belong to distinct lineages and show differences in their mode of activation, they present striking similarities which include the ability to release massive amounts of cytokines with extreme celerity and without prior sensitization. Like NK cells, NKT cells constitutively express mRNA but not protein for IFN-γ, a hallmark of their poised effector stage (5
). Unlike NK cells, however, NKT cells also produce TNF-α, IL-4 and IL-13 and the importance of Th1 and Th2 cytokines has been demonstrated in vivo
, in conditions where NKT cells either improved or aggravated disease (6
). NKT-deficient CD1d- or Jα18- knockout mice suffer from impaired early IFN-γ secretion, correlating during some infections with reduced anti-microbial defense (6
), while ablation of NK cells impairs immune responses preferentially to Shigella flexneri
and Mycobacterium tuberculosis
NKT cells interact with and possibly regulate NK cells during infections in multiple ways (7
). NK cell activation contributes to the local and systemic inflammation during lipopolysaccharide (LPS)-induced shock (10
), whereas NKT cells may even exhibit anti-inflammatory effects (11
). The rapid cross-activation of NK cells upon NKT cell activation (12
) implies that innate lymphocyte responses can augment each other dramatically under certain circumstances and in response to defined stimuli. However, the engagement of different receptors may contribute to the opposing effects of NKT cells in the respective models and may interfere with NK cell activation in various ways. Thus, these receptors and their respective effects on NKT and NK cells need to be identified and their mode of action on both cell populations delineated.
Sepsis is associated with a strong activation of the complement system and the generation of the anaphylatoxins (ATs) C3a and C5a in mice and humans(13
). Both ATs exert their biologic functions through binding and activation of their cognate G-protein-coupled receptors, i.e. the C3a receptor (C3aR) and the C5a receptor (C5aR/CD88). C5a and its primary degradation product, C5adesArg, can further bind to another seven-transmembrane receptor, C5a receptor-like 2 (C5L2), which is uncoupled from G-proteins (13
). In CLP-induced septic peritonitis, functional roles for both C5a receptors have been demonstrated (14
). However, as this is a model involving intestinal flora, the effects of defined bacterial species on the expression of C5aR and C5L2 and the subsequent effects of C5aR and C5L2 expression on the control of bacterial infection and the release of cytokines remain unknown. Up to now, the detrimental effects of C5a in sepsis have mainly been attributed to the paralysis of neutrophils (15
). However, other cell populations may be involved in C5a mediated effects. As innate lymphocytes are not only a major cellular source of various cytokines, but also shape the subsequent adaptive immune response we have assessed in this study the role of C5a in the activation of NK and NKT cells in an E. coli
-induced sepsis model.
We observed that NKT and NK cells from naïve mice already express both C5a receptors, C5aR and C5L2 at the mRNA, but not at the protein level. In vivo exposure to E. coli resulted in the rapid surface expression of C5aR protein on subpopulations of NKT and NK cells, which was associated with an enhanced expression of the early activation marker CD69.
C5aR deficiency resulted in a decreased expression of Nkp46 on NK cells and in a reduced release of IFN-γ and TNF-α by NKT and NK cells along with an impaired recruitment of NKT and NK cells to the site of infection. Animal survival in response to E. coli-induced sepsis was significantly higher in the absence of C5aR, NKT and NK cells. Improved survival correlated with reduced IFN-γ and TNF-α levels suggesting that signaling through C5aR during sepsis contributes to the detrimental effects of NKT and NK cells. Sepsis induction in C5aR+/C5aR− mixed bone marrow chimeras identified cognate engagement of C5aR on NKT cells as a critical factor for NKT cell recruitment, while NK cell recruitment required preferentially C5aR engagement on DCs.