P. aeruginosa produce N-3-oxododecanoyl homoserine lactone (3OC12-HSL), a so-called “quorum-sensing molecule” that provides signals for the production of virulence factors and for bacterial biofilm formation in a paracrine manner. We now found that 3OC12-HSL, but not its 3-deoxo-isomer or acyl homoserine lactones with shorter fatty acids, is also able to activate human polymorphonuclear neutrophils (PMN) in vitro: 3OC12-HSL enhanced the phagocytosis of opsonised bacteria in vitro; up-regulated the surface expression of phagocytosis-related receptors, and was chemotactic for PMN. Because induction of chemotaxis implies the polarisation of the cell by receptors expressed on the surface, we performed uptake studies with radiolabelled 3OC12-HSL. At 4° C we found saturable binding of the radiolabelled 3OC12-HSL, which could be inhibited by an excess of unlabelled 3OC12-HSL, indicating specificity of binding, and hence expression of a distinct surface receptor. By use of selective inhibitors, a signalling pathway, comprising phosphotyrosine kinases, phospholipase C, protein kinase C, mitogen activated protein kinase C was delineated, but in contrast to the well-studied chemokines C5a and interleukin 8, the chemotaxis in response to 3OC12-HSL did not depend on pertussis toxin-sensitive G proteins. Selective surface receptors for 3OC12-HSL have been identified in various bacteria species, but scrutinising a human gene bank did not reveal homologous structures. While the characterisation of the surface receptor awaits further studies, the functional consequence of the cross-kingdom signalling is obvious: by recognising and responding to 3OC12-HSL PMN are attracted to the site of a developing biofilm, and thus may prevent its progression and by that persistent infection.

Implant-associated osteomyelitis is caused by persistent bacterial infections, predominantly by staphylococci species forming biofilms on implants or osteosynthesis – materials. In the majority of patients the systemic immune response appears to be inconspicous with only minor upregulation of activation-associated receptors on the polymorphonuclear neutrophils (PMN). We found, however, evidence the activation of T cells, apparent as the expansion of CD4+ and CD8+ T cells bearing the activation-associated receptor CD11b. These cells also lacked the co-stimulatory molecule CD28, which is a further indicator for T cell activation. Moreover, small populations of T cells expressing Toll-like receptors (TLR)1, TLR2 or TLR4 were detected in the patients, while in healthy donors less than 1 % of T cells express TLR. A preferential association of TLR1- and TLR2-expression with CD28-CD11b+ cells was seen, compatible with the fact these cells represent an activated phenotype. In addition to the peripheral blood we also analysed leukocytes recovered from the infected site during surgery for removal of the implant. Predominantly PMN were found, highly activated as judged from their surace recpetor pattern, but also CD4+ and CD8+ T cells. As expected, these T cells represented an activated phenotype, and particularly the CD8+ cells expressed CD57, a receptor identifying end-differentiated T cells. The T cells recovered from the infected site, but not the peripheral blood T cells, produced interferon gamma, a cytokine known to support the function of phagocytic cells. In conclusion our data provide evidence that in response to local, persistent bacterial infections T cells are activated to acquire – among others – receptors selective for bacterial products, which in turn might modulate the T cell function and hence the host defence.