In vitro models of hematogenously disseminated candidiasis

In vitro models, in which Candida albicans is incubated with one or more type of host cell, are a powerful tool for dissecting the pathogenesis of infection. Using these models, investigators have elucidated important responses of both the host and the fungus that influence the outcome of infection. In addition, tissue culture models have been invaluable for discovering host cell receptors for C. albicans, their cognate fungal ligands, and downstream signaling pathways. Furthermore, in vitro models can serve as surrogates for in vivo infection, facilitating the discovery of key host defense mechanisms and fungal virulence factors. Although tissue culture models of infection have numerous strengths, they also have weaknesses that preclude them from completely replacing animal models of infection. C. albicans is an opportunistic pathogen that typically requires dysfunction of the host innate immune system to cause invasive infection. Because the interactions of C. albicans with the innate immune system are a key factor in determining the outcome of infection, the interactions of C. albicans with professional phagocytes has been modeled using macrophage cell lines. For example, Lorenz and Fink used the J774 mouse macrophage cell line to discover that the glyoxylate cycle plays a key role in C. albicans virulence. 1 Macrophage cell lines have also been invaluable for identifying the receptors that enable phagocytes to recognize specific components of the C. albicans cell wall. Receptors for C. albicans that have been discovered using macrophage cell lines include dectin-1, 2 dectin-2, 3 mincle, 4 galectin-3, 5 the macrophage mannose receptor, 6 and integrin α M β 2 . 7 Notably, the importance of these receptors for the host defense against disseminated candidiasis in vivo has been confirmed using mutant strains of mice that lack these receptors. Furthermore, studies of humans with a naturally occurring stop mutation in the DECTIN-1 gene demonstrate that dectin-1 is necessary for the host defense against mucosal candidiasis. 8 This strong correlation between in vitro and in vivo results supports the utility of the macrophage model for studying the interactions between C. albicans and professional phagocytes. A key issue with macrophage studies is the source of the cells. The mouse J774 and RAW 264.7 macrophage cell lines are particularly useful for in vitro studies because they are wellcharacterized, numerous mutant strains are available, and they are relatively easy to transform. However, a limitation of these cell lines is that they are very poor at killing C. albicans. This limitation can be overcome by using primary mouse macrophages, either derived from bone marrow cells or elicited from the peritoneum. Because these macrophages are from mice, they are highly likely to be predictive of the interactions of C. albicans with phagocytes during invasive infection in these animals. Nevertheless, mouse macrophages differ significantly from their human counterparts. 9 For example, mouse macrophages produce high amounts of nitric oxide (NO), which contributes to microbial killing, whereas human macrophages produce much lower levels of NO and kill microorganisms by NO-independent mechanisms. 10 Because of the differences between mouse and human macrophages, it is important to use human macrophages, preferably primary cells, to verify results obtained with mouse macrophages.