Identification of heat shock protein 60 as the ligand on Histoplasma capsulatum that mediates binding to CD18 receptors on human macrophages

It has been previously demonstrated that Histoplasma capsulatum (Hc), a facultative, intracellular parasite, binds to the CD18 family of receptor glycoproteins (CD11a/CD18 {LFA-1}, CD11b/CD18 {CR3}, CD11c/CD18 {CR4}) on host macrophages via an unknown ligand(s). Purified human CR3 was used to probe a farwestern blot of a detergent extract of Hc cell wall and cell membrane. CR3 identified a single, 60-kDa protein, which was subsequently identified as heat shock protein 60 (HSP60).;Cell surface biotinylation of viable yeasts followed by immunoprecipitation with streptavidin beads, and visualization by western blotting provided evidence that HSP60 existed on the yeast cell surface. Subsequent flow cytometric analysis, electron microscopy, and confocal microscopy confirmed Hc HSP60 surface expression.;Recombinant HSP60 inhibited that attachment of Hc yeasts to macrophages or CR3-expressing CHO cells in a concentration-dependent fashion. Polystyrene beads coated with recombinant HSP60 bound avidly to macrophages, and preincubation of macrophages with a cocktail of antibodies to the CD18 alpha chains inhibited binding of HSP60-beads. Freeze/thaw lysate (FITS), a crude extract of Hc surface proteins was found to contain HSP60, and potently inhibited the attachment of Hc to macrophages. Depletion of HSP60 from F/TE by affinity chromatography completely removed the capacity of F/TE to block the binding of Hc to macrophages. Recombinant HSP60 did not inhibit the binding of Hc to dendritic cells (DC), which bind Hc via the fibronectin receptor VLA-5. Moreover, F/TE inhibited the attachment of Hc to DC, even when depleted of HSP60. Thus, Hc HSP60 appears to be the primary ligand that mediates attachment of Hc to CD18 receptors on macrophages, but not to VIA-5 on dendritic cells.