The multiplicity of infection does not affect interactions between Cryptococcus neoformans and macrophages

Major determinants of the outcome of infection include growth of the pathogen and response of immune cells such as macrophages. Cryptococcus neoformans is a fungal pathogen which may grow within the extracellular environment, or may exploit host macrophages as a niche for replication and dissemination. The clinical outcome of cryptococcal infection varies widely between individuals even when key host and pathogen molecular factors are the same. For a broad range of infections altering pathogen density is known to influence progression and outcome of infection by affecting immune response and pathogen biology (e.g. via innate immune signalling or microbial quorum sensing). Here, using time lapse imaging of murine cell line and human primary macrophages in vitro, we examined the effect of altering pathogen density on the interactions of macrophages with cryptococci. We find that increasing fungal burden over several orders of magnitude did not increase or decrease phagocytosis by murine J774 macrophage-like cells or human monocyte-derived macrophages, illustrating neither dose-dependent immune activation nor dampening of the phagocytic response. Furthermore, increasing fungal density alone was not sufficient to alter the ability of cryptococci to grow intracellularly and has no significant effect on fungal doubling time for cryptococci that were intracellular or extracellular. This suggested that individual macrophage-Cryptococcus interactions were not affected by even large changes in fungal density, an important finding in understanding what determines the outcome of cryptococcal infection.