What’s so special about Giardia ventral flagella? Interspecies cross-reacting monoclonal antibody againstPneumocystis jirovecireacts with cilia and sparks questions on host-pathogen interactions

A mouse monoclonal antibody (Moab 4B8) cross-reacting with cilia/flagella was obtained by immunization with Pneumocystis-infected human lung tissue. A key observation was that Moab 4B8 reacted with the ventral flagella of Giardia intestinalis , but not with the three other flagellar pairs of this protozoan. To further identify the 4B8 target, its distribution was studied by immunofluorescence staining of cells and tissues of various origin. The target epitope recognized by Moab 4B8 was found to be associated with structures rich in microtubules; e.g. the mitotic spindle of cultured cells, ciliated airway epithelia, Sertoli cells of the testis and ependymal cells lining brain ventricles. The conserved nature of the 4B8 target was further shown by its presence in cilia of metazoan Schistosome larva and the green alga Chlamydomonas reinhardtii . Absence of the 4B8 target from Trypanosomes and Leishmania flagella suggested that it is involved in some function not primarily related to motility. Its presence in only the ventral flagella of Giardia therefore provides a unique opportunity to elucidate the relationship between ciliary structure and function in the same organism. The observed locations of the 4B8 target in tissues and cells of various origin, suggest a similarity to annexins - and specifically to α-19-giardin. This raises the possibility that it is involved in intra-flagellar transport and provides a basis for further studies aiming at its identification. Author Summary Pneumocystis is a ubiquitous fungal organism apparently colonizing the lung at an early age to cause pneumonia only in individuals with an impaired immune system. In the alveolar spaces of such individuals, extensive and frequently fatal proliferation of the pathogen occurs. Pneumocystis has no known reservoir in nature and apparently is transmitted directly from infected individuals via an airborne route. Adaptation of this Ascomycotic fungus to a parasitic lifestyle during its evolution apparently resulted in dependence upon host nutrients, but little is known about this presumed adaptation process. In this report, a previously unrecognized constituent of human Pneumocystis is detected using a monoclonal anti- Pneumocystis jiroveci antibody (Moab 4B8) which was obtained as a by-product in the search for reagents useful in diagnostics. The Moab 4B8 was shown to react with Pneumocystis but also with cytoskeletal microtubules, e.g. in ciliated epithelia, but not ubiquitously a constituent of the conserved cilia/flagella axonemal structure. A striking example of the discriminating capacity of antibody 4B8 was seen in immunofluorescent staining of the protozoan Giardia intestinalis , where only one out of four flagellar pairs expresses the target epitope. This observation of flagellar heterogenicity provoked the question raised in the title of this report. It also provides the basis for the discussion, which arrives at suggestive evidence for the involvement of the described evolutionarily conserved target in host-pathogen interactions related to membrane transport.