Candida albicans infection inhibits macrophage cell division and proliferation
Candida albicans is a major life-threatening human fungal pathogen.
Host defence against systemic infection relies mainly on phagocytosis of Candida by cells of the innate immune system (Gow et al., 2012).
Our recent work has studied C. albicans-macrophage interactions using phagocytosis assays and live cell video microscopy coupled with sophisticated image analysis tools (McKenzie et al., 2010; Lewis et al., 2012).
Such studies reveal dynamic aspects of the host-pathogen interaction that are not evident from studies based on fixed time point analyses. We have shown, for example, that C. albicans can rarely use non-lytic expulsion as a means to escape from macrophages, leaving both pathogen and host cell viable (Bain et al., 2012) and provided detailed information on how hyphal formation within macrophages leads to macrophage lysis and C. albicans escape (McKenzie et al., 2010). Here we show that infection of macrophages with C. albicans can result in failure of macrophages to complete mitosis. Instead of separating into two individual cells, daughter phagocytes remain together and can fuse to reform a single large macrophage. This intriguing and surprisingly common phenomenon may represent yet another virulence attribute of C. albicans.
2. Description of observation
Video 1 shows an example of a J774.1 macrophage that successfully underwent mitosis. When macrophages were cultured with live C. albicans, 29.5 ± 5.7% of macrophages initiated mitosis; however, 35.9 ± 6.1% of 147 mitosis events examined resulted in failed cell division. In Video 2 the macrophage ingests multiple C. albicans, which form hyphae within the macrophage. The host macrophage initiated mitosis, but instead of completely separating, the daughter cells remained fused together. In all cases of failed cell separation, C. albicans hyphae were observed spanning both daughter cells. Macrophages did not persist in their attempt to undergo mitosis indefinitely. Instead macrophages fused back together on average 44.6 ± 3.3 min after initiation of cytokinesis. It is noteworthy that phagocytosis could still occur whilst macrophages were attempting to undergo mitosis.
Interestingly, infection with C. neoformans has also been shown to promote cell cycle progression as a strategy to spread infection (Luo et al., 2012).
Failure to complete cell division in macrophages infected with hyphal C. albicans may occur due to difficulties in dividing large cargos, such as hyphae, between daughter cells and our observations suggest that hyphal size may be an important factor in this process.
C. albicans hyphae may impede spindle formation or prevent the actin ring from contracting and pinching the macrophage into separate daughter cells during cytokinesis.
Some microbial pathogens are known to interfere with cell cycle progression, including Clamydia trachomatis, which can selectively block cytokinesis (Greene and Zhong, 2003). Likewise, C. albicans may either have evolved a specific mechanism to prevent macrophage replication or interfere with this process indirectly due to the enlargement and distension of the phagosome.
The observation that C. albicans hyphae spanned both daughter cells in all cases of failed cell separation implies mechanical rather than active inhibition of cytokinesis. Interestingly, non-lytic expulsion of hyphalC. albicans has been observed immediately prior to a host macrophage undergoing mitosis (Bain et al., 2012). It is possible that, in this instance, expulsion of C. albicans may have enabled mitosis and cytokinesis to proceed normally.
Thus, the ability of C. albicans to impede daughter cell separation may be yet another mechanism employed by C. albicans to aid its survival. An alternative explanation is that this process benefits the host, in that macrophages may sense that they carry dividing fungal cells and inhibit division to prevent spreading of the infection. Interestingly, we observed failed cell division most frequently in clusters of macrophages exposed to and infected with multiple C. albicans hyphae. One may speculate that this may result in macrophage giant cell formation that may help suppress growth of large sized microbial cells. Failed cell division is likely to have profound consequences on the host response to infection with C. albicans and future studies are needed to establish the mechanism by which phagocytosis of fungal cells inhibits macrophage mitosis.