Cyst rupture in immunocompetent hosts

One of the clinical problems in immunocompro-mised hosts is recrudescence of the infection resulting in stage conversion back to actively proliferating and tissue-destructive tachyzoites. To examine the situation in the immunocompetent host, the brains of immunocompetent chronically infected mice have been examined. It is found that, indeed, a very small percentage of tissue cysts are rupturing at any given time during chronic infections (Ferguson et al., 1989). The initial change appears to be the death of the host cell. With the exposure of the parasite antigen in the cyst wall, there is evidence of a rapid and massive cell-mediated immune response involving numerous inflammatory cells (monocytes and even neutrophils). These are observed around the still apparently intact cyst (Figure 2.25A). With the rupture of the cyst wall, there is a further influx of macrophages into the cyst (Figure 2.25C). The macrophages phagocytose the bradyzoites, where there is fusion with lysosomes resulting in the formation of phagolysosomes and destruction of the parasites (Figure 2.25B). This immune response results in the formation of small inflammatory lesions (microglia nodules) with some evidence of host-cell apoptosis but limited host-tissue damage. The bradyzoites appear to be destroyed before they can undergo replication or

0 0

Post a comment