Survival of protozoan intracellular parasites in host cells

Abstract

The most common human diseases are caused by pathogens. Several of these microorganisms have developed efficient ways in which to exploit host molecules, along with molecular pathways to ensure their survival, differentiation and replication in host cells. Although the contribution of the host cell to the development of many intracellular pathogens (particularly viruses and bacteria) has been unequivocally established, the study of host-cell requirements during the life cycle of protozoan parasites is still in its infancy. In this review, we aim to provide some insight into the manipulation of the host cell by parasites through discussing the hurdles that are faced by the latter during infection.

Figure 1

Recognition, entrance and survival: a schematic representation of some of the strategies adopted by pathogens. (A) Induced recruitment of lysosomes to fuse with the host-cell plasma membrane to form the required vacuole for Trypanosoma cruzi. (B) A parasite, such as Toxoplasma, uses its own form of motility to propel itself into the host cell. (C) Once inside, intracellular pathogens follow different routes: they either reside inside the vacuole (i), which sometimes fuses with the exocytic/endocytic pathways (ii), or they reside in the host cytosol (iii). The same pathogen might use different strategies at different stages of development. (D) Independent of the route taken, all pathogens face similar problems, which include how to ensure that the cell is alive or dead at the appropriate times, how to acquire nutrients, how to avoid host-recognition systems and how to increase in size. Parasites and their proteins are shown in orange. E, endosomes; Ly, lysosomes.

Clockwise from top left: Sónia S. Albuquerque, Maria M. Mota, Patricia Leirião & Cristina D. Rodrigues