Giardia lamblia: traffic of a trophozoite variant surface protein and a major cyst wall epitope during growth, encystation, and antigenic switching.

Both trophozoites and cysts of Giardia lamblia have unique outer surfaces that protect them from very different hostile environments. However, little is known about the transport of these important molecules to the cell surface. We used monospecific anti-recombinant TSA 417 antibodies and mAb 8C5 in double label immunoelectron microscopy to compare the localization and transport of this major trophozoite surface antigen (TSA) with that of a prominent cyst wall epitope during vegetative growth, encystation, and antigenic switching in vitro. TSA 417 is a marker of the constitutive transport of the major plasma membrane protein, while the 8C5 epitope traces a differentiation-regulated secretory pathway to the cyst wall. Both proteins localized to the nuclear envelope endoplasmic reticulum (ER) cisternae, ER, and cytoplasmic membrane cisternae, reflecting their site of synthesis, but only the differentiation-specific epitope 8C5 localized to the encystation-specific vesicles (ESV). These large secretory vesicles form only during encystation and transport cyst antigens to the nascent wall. In contrast, only TSA 417 was found on the outer surface of the plasmalemma of trophozoites and encysting cells and underlaying the walls of many cysts, while only 8C5 localized to the cyst wall. As encystation progressed, TSA 417 disappeared from the plasmalemma and increased in the lysosome-like PV and other large cytoplasmic vesicles. In contrast to their segregation in the ESV and on the cell surface, both TSA 417 and 8C5 were found in the peripheral vesicles, presumably an endocytic compartment. We propose that this may be the initiation of a stage in differentiation-driven antigenic switching of TSA 417, in which this antigen is no longer synthesized or exported to the plasmalemma, but is taken back inside the cell.