Pneumocystis Carinii Infection And Immunity

Sue A Theus and Peter D Walzer, Cincinnati Veterans Affairs Medical Center and Division of Infectious Diseases, Department of Internal Medicine, University of Cincinnati College of Medicine, Cincinnati, Ohio, USA

Copyright © 1998 Elsevier Ltd. All Rights Reserved.

Pneumocystis carinii is an extracellular pathogen that causes pneumonia in the immunocompromised host. Pneumocystosis is one of the leading opportunistic infections and causes of death in patients with the acquired immune deficiency syndrome (AIDS). No reliable in vitro culture system exists for P. carinii, which effectively hampers research on this pathogen. Until recently, the taxonomic status of this organism, based on morphological and ultrastructural characterization, was not clearly defined. However, molecular studies have shown that P. carinii is more closely related to fungi than protozoa. This classification was initially based on DNA homology studies using a single RNA ribosomal gene, but was subsequently confirmed by analysis of mitochondrial genes and protein coding sequences.

Two predominant forms of the organism have been identified: the trophozoite and the cyst. The trophic form is the smaller form (1-4 pm), pleomorphic, and exists in clusters. The cyst stage is larger (5-8 pm) and contains up to eight intracystic bodies. Two prominent groups of P. carinii antigens have been identified. A large surface complex, termed major surface glycoprotein (MSG), gpA or gpl20 has been found in organisms derived from humans and other animals with a reported molecular weight of 95-140 kDa under reducing conditions. MSG is encoded by a family of genes, suggesting the possibility of antigenic variation. N-Linked carbohydrates comprise about 5-10% of the molecular mass of this glycoprotein. A major function of MSG is to mediate the interaction of P. carinii with the host. The other major antigen complex is a glycoprotein that migrates as a broad band of 45-55 kDa in rat P. carinii and 35-45 kDa in human P. carinii. The gene encoding the rat P. carinii 45-55 kDa antigen has recently been cloned but awaits further characterization.

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