A co-stimulatory signal through ICAM-beta2 integrin-binding potentiates neutrophil phagocytosis.

The beta2 integrin LFA-1 (lymphocyte function associated antigen; CD11a/CD18) is the common ligand for the intercellular adhesion molecules (ICAMs). Integrins support cell function by providing co-stimulatory second signals that are a precondition for full cell activation first described for ICAM-1-binding to LFA-1 in lymphocytes. Integrins can also serve to activate functions associated with distinct subunits of other integrins. In addition to LFA-1, neutrophils express the beta2 integrin Mac-1 (CD11b/CD18; CR3) that apparently contains multiple sites that bind invading microbes directly or through surface-fixed C3, resulting in activation of the phagocyte function. Expression of the LFA-1 counter-receptor ICAM-1 on endothelial cells occurs only at the site of inflammation. Therefore, in neutrophils, ICAM-1 ligand binding could, as with lymphocytes, also play a part as a co-stimulatory signal to induce full phagocytotic function. We show that in neutrophils, the LFA-1 ligand interaction is the stimulatory signal to express full phagocytotic activation. This is best demonstrated by the rapid association of Streptococcus pyogenes with neutrophils, followed by ingestion, strong oxidative-burst induction and enhanced killing of these bacteria, which are well-known for their resistance to human neutrophil defense. These findings may contribute to the development of therapeutic strategies targeting the modulation of ICAM-1-leukocyte interaction.

Analyses of phagocytosis, evoked oxidative burst, and killing of black yeasts by human neutrophils: a tool for estimating their pathogenicity?

The pathogenicity of several dematiaceous yeasts that have, to date, rarely been isolated in humans remains unclear. Because professional phagocytes are prominent in lesions caused by dematiaceous fungi, we address this issue by comparing phagocytosis, evoked oxidative burst and killing by human neutrophils of different black yeasts in vitro. Whereas phagocytosis of all black yeasts tested and evoked oxidative burst yielded comparable results, in contrast, the degree of killing differed significantly after 5 h. Thereby, two groups could be identified; one in which strains are killed at high rates, for example, Hortaea werneckii (81 +/- 11.6%), Exophiala castellanii (96 +/- 8.6%), Phaeoannellomyces elegans (93 +/- 9.7%), Phaeococcomyces exophialae (87 +/- 8.7%), and the other in which strains are killed to a lesser degree, for example, Exophiala dermatitidis (ATCC 34100) (61 +/- 9.5%), E. dermatitidis (CBS 207.35) (66 +/- 7.5%), E. jeanselmei (50 +/- 10.5%), E. mesophila (63 +/- 11.6%), E. bergeri (63 +/- 9.1%), and E. spinifera (57 +/- 9.6%). Non-pigmented yeasts were killed at levels comparable with those at which the white mutant strain of E. dermatitidis (ATCC 44504) was killed (95 +/- 7.5%); the yeast strains tested were Candida albicans (DSM 11943) (95 +/- 4.0% killing) and Saccharomyces cerevisiae (DSM 1333) (95 +/- 10.3%). Comparison of killing rates with the observed pathogenicity of the melanized species suggests that low killing rates might indicate or even predict a high degree of invasiveness. Although previous experiments revealed that melanization conferred killing resistance on E. dermatitidis, the differences in killing rates of other dematious fungi suggest that melanization of the cell wall is in itself insufficient to confer virulence.