Endoplasmic reticulum (ER) stress is a condition in which the protein folding capacity of the ER becomes overwhelmed by an increased demand for secretion or by exposure to compounds that disrupt ER homeostasis. In yeast and other fungi, the accumulation of unfolded proteins is detected by the ER-transmembrane sensor IreA/Ire1, which responds by cleaving an intron from the downstream cytoplasmic mRNA HacA/Hac1, allowing for the translation of a transcription factor that coordinates a series of adaptive responses that are collectively known as the unfolded protein response (UPR). Here, we examined the contribution of IreA to growth and virulence in the human fungal pathogen Aspergillus fumigatus. Gene expression profiling revealed that A. fumigatus IreA signals predominantly through the canonical IreA-HacA pathway under conditions of severe ER stress. However, in the absence of ER stress IreA controls dual signaling circuits that are both HacA-dependent and HacA-independent. We found that a ΔireA mutant was avirulent in a mouse model of invasive aspergillosis, which contrasts the partial virulence of a ΔhacA mutant, suggesting that IreA contributes to pathogenesis independently of HacA. In support of this conclusion, we found that the ΔireA mutant had more severe defects in the expression of multiple virulence-related traits relative to ΔhacA, including reduced thermotolerance, decreased nutritional versatility, impaired growth under hypoxia, altered cell wall and membrane composition, and increased susceptibility to azole antifungals. In addition, full or partial virulence could be restored to the ΔireA mutant by complementation with either the induced form of the hacA mRNA, hacA i, or an ireA deletion mutant that was incapable of processing the hacA mRNA, ireA Δ10. Together, these findings demonstrate that IreA has both HacA-dependent and HacA-independent functions that contribute to the expression of traits that are essential for virulence in A. fumigatus.

An in vitro system to investigate the ability of macrophages to recognize and ingest senescent polymorphonuclear neutrophils has been used that uses chromium-labeled neutrophils and staining for myeloperoxidase (MPO). Human monocyte-derived macrophages obtained from in vitro cultures were able to recognize "aged" but not freshly isolated 51Cr-labeled human neutrophils and ingest them. Freshly isolated monocytes did not exhibit this property. Because the aged neutrophils were greater than 95% viable, death did not appear to be a prerequisite for recognition and ingestion. Serum was not required for the aging of the neutrophils, and when serum was used, different concentrations did not appear to effect the aging process; that is, neutrophils aged in different concentrations of serum were ingested equally. Phagocytosis of senescent neutrophils by macrophages occurred in a time-dependent manner and was also dependent on the number of neutrophils added. Monocyte-derived macrophages first exhibited the ability to phagocytose senescent neutrophils on the 3rd d of culture, with the percentage of active macrophages increasing through day 7. In experiments with rabbit mononuclear phagocytes, immune complex-induced inflammatory macrophages from the lung but not resident bronchoalveolar macrophages or peripheral blood monocytes were found to be capable of recognition and ingestion of senescent rabbit neutrophils. These data suggest that the monocyte maturation process, akin to that seen during inflammation, is necessary in vitro before macrophages recognize and remove senescent neutrophils.

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Histoplasma capsulatum (Hc), is a facultative intracellular fungus that binds to CD11/CD18 receptors on macrophages (Mφ). To identify the ligand(s) on Hc yeasts that is recognized by Mφ, purified human complement receptor type 3 (CR3, CD11b/CD18) was used to probe a Far Western blot of a detergent extract of Hc cell wall and cell membrane. CR3 recognized a single 60-kDa protein, which was identified as heat shock protein 60 (hsp60). Biotinylation of viable yeasts, followed by precipitation with streptavidin-coated beads, and Western blotting with anti-hsp60 demonstrated that hsp60 was on the surface of Hc yeasts. Electron and confocal microscopy revealed that hsp60 resided on the yeast cell wall in discrete clusters. Recombinant hsp60 (rhsp60) inhibited attachment of Hc yeasts to Mφ. Recombinant hsp60 and Abs to CD11b and CD18 inhibited binding of yeasts to Chinese hamster ovary cells transfected with CR3 (CHO3). Polystyrene beads coated with rhsp60 bound to Mφ, and attachment was inhibited by Abs to CD11 and CD18. Freeze/thaw extract (F/TE), a preparation of Hc yeast surface proteins that contained hsp60, inhibited the attachment of Hc yeasts to Mφ. Depletion of hsp60 from F/TE removed the capacity of F/TE to block binding of Hc to Mφ. Interestingly, rhsp60 did not inhibit binding of Hc yeasts to dendritic cells (DC), which recognize Hc via very late Ag 5. Moreover, F/TE inhibited attachment of Hc to DC even when depleted of hsp60. Thus, Hc hsp60 appears to be a major ligand that mediates attachment of Hc to Mφ CD11/CD18, whereas DC recognize Hc via a different ligand(s).