TOXOPLASMA-GONDII BRADYZOITES FORM SPONTANEOUSLY DURING SPOROZOITE-INITIATED DEVELOPMENT

Tachyzoites (VEG strain) that emerge from host cells infected with Tox oplasma gondii sporozoites proliferate relatively fast and double thei r number every 6 h, This rate of growth is intrinsic, as neither the n umber of host cells invaded nor host cell type appears to influence em ergent tachyzoite replication. Fast tachyzoite growth was not persiste nt, and following similar to 20 divisions, the population uniformly sh ifted to slower growth. Parasites 10 days post-sporozoite infection do ubled only once every; 15 h and, unlike emergent tachyzoites, they gre w at this slower rate over several months of continuous cell culture. The spontaneous change in tachyzoite growth rate preceded the expressi on of the bradyzoite-specific marker, BAG1, Within 24 h of the growth shift, 2% of the population expressed BAG1, and by 15 days post-sporoz oite infection, 50% of the parasites were positive for this marker. Sp ontaneous BAG1 expression was not observed in sporozoites or in tachyz oites during fast growth (through day 6 post-sporozoite inoculation), although these tachyzoites could be induced to express BAG1 earlier by culturing sporozoite-infected cells at pH 8.3. However, alkaline trea tment also reduced the replication of emergent tachyzoites to the rate of growth-shifted parasites, supporting a link between reduced parasi te growth and bradyzoite differentiation. The shift to slower growth w as closely correlated with virulence in mice, as the initially fast-gr owing emergent tachyzoites were avirulent (100% lethal dose, >10(4) pa rasites), while a mutant VEG strain (MS-J) that is unable to growth sh ift caused 100% mortality in mice inoculated with 10 parasites, Parasi tes recovered from gamma interferon knockout mice inoculated with emer gent tachyzoites grew at a slow rate and expressed BAG1, confirming th at the replication switch occurs in animals and in the absence of a pr otective immune response.