Antibodies to a cell surface histone-like protein protect against Histoplasma capsulatum

Abstract

A protective role for antibodies has not previously been described for host defense against the pathogenic fungus Histoplasma capsulatum (Hc). Mouse mAb's were generated from mice immunized with Hc yeast that binds the cell surface of Hc. Administration of mAb's before Hc infection reduced fungal burden, decreased pulmonary inflammation, and prolonged survival in a murine infection model. Protection mediated by mAb's was associated with enhanced levels of IL-4, IL-6, and IFN-γ in the lungs of infected mice. The mAb's increased phagocytosis of yeast by J774.16 cells through a CR3-dependent process. Ingestion of mAb-opsonized Hc by J774.16 macrophage-like cells was associated with yeast cell growth inhibition and killing. The mAb's bound to a 17-kDa antigen expressed on the surface of Hc. The antigen was identified as a histone H2B-like protein. This study establishes that mAb's to a cell surface protein of Hc alter the intracellular fate of the fungus and mediate protection in a murine model of lethal histoplasmosis, and it suggests a new candidate antigen for vaccine development.

Antibody Therapy for Histoplasmosis

“…Displayed on the surface are a variety of proteins that have been associated with virulence or utilized in diagnosis of the fungus (Figure 1). MAbs have been generated that bind H. capsulatum cell wall antigens, including melanin (Nosanchuk et al, 2002), histone 2B (Nosanchuk et al, 2003), Hsp60 (Guimaraes et al, 2009), M antigen (Guimaraes et al, 2008), and a 70-kDa protein (Lopes et al, 2010). …”

“…Immunization with heat-killed H. capsulatum yeast cells resulted in the generation of a panel of IgM isotype mAbs that specifically bound cell surface expressed histone 2B (Nosanchuk et al, 2003). Although the identification of histone 2B as a cell surface antigen was surprising, histones are increasingly described on the cell surface of diverse host cells as well as pathogens; such as the cell surface of Mycobacterium leprae (Pessolani et al, 1993; Marques et al, 2000, 2001) and Mycobacterium smegmatis (Pethe et al, 2001) that are associated with binding to host cells and, specifically in the case of M. leprae , cell invasion (Shimoji et al, 1999).…”

“…The protective effect of these mAbs was associated with an alteration in the intracellular fate of H. capsulatum (Nosanchuk et al, 2003; Shi et al, 2008). The mAbs reduced the capacity of the fungus to replicate intracellularly.…”

Surface architecture of Histoplasma capsulatum

“…Similarly Mycobacterium smegmatis binds to laminin on human pneumocytes and macrophages through a histone-like protein located on its surface (Pethe et al, 2001). We have described a histone 2B-like protein of 17-kDa antigen expressed on the surface of Hc, and it is speculated that the protein is important in cell–cell signaling and modulation of the immune response by the fungus (Nosanchuk et al, 2003). …”

“…The protective response mediated by these mAbs was associated with enhanced levels of IL-4, IL-6, and IFN-γ in the lungs of infected mice. The mAbs increased phagocytosis of yeast by J774.16 cells through a CR3-dependent process and uptake of the opsonized yeast was associated with yeast growth inhibition and killing (Nosanchuk et al, 2003; Nosanchuk, 2005). The altered intracellular fate of the opsonized Hc yeast was characterized by significantly enhanced macrophage phagosome activation and maturation and also a reduced ability of the organism to regulate the phagosomal pH.…”