Cloning and characterization of a repetitive sequence from Pneumocystis carinii

Cell wall antigens of Pneumocystis carinii trophozoites and cysts: purification and carbohydrate analysis of these glycoproteins

We have purified and biochemically analyzed individual cell wall glycoproteins of Pneumocystis carinii. Our results show that corresponding core glycoproteins constitute the cell wall antigens in both trophozoites and cysts, and glycosylation of these glycoproteins does not appear to be significantly altered during development. Cysts and trophozoites in rat-derived organism preparations were separated from each other by counterflow centrifugal elutriation, then treated with Zymolyase to obtain the cell wall fractions. Gel electrophoresis patterns of these fractions from both life-cycle stages were qualitatively similar. Ten major antigenic glycoproteins in these fractions were purified by preparative continuous elution gel electrophoresis. All ten glycoproteins from cysts and trophozoites contained mannose, glucose, galactose, and N-acetylglucosamine, and some contained traces of fucose. The glycoproteins of cysts had more mannose than their trophozoite counterparts. The trophozoite glycoproteins differed from those of the cyst by the presence of xylose. To examine the species-specificity of glycoprotein glycosylation, preparations of human-derived P. carinii (comprised of mixed life-cycle stages) were also examined and found to contain the same sugars as those found in rat-derived organisms. Most of the purified rat-derived glycoproteins bound Concanvalin A, which was abolished by treatment with N-glycanase. This suggested that the majority of the oligosaccharides were N-linked to the proteins, but attempts to identify carbohydrate linkage sites by amino acid sequencing were hampered by apparent modifications of residues. The peptides derived by cyanogen bromide cleavage revealed distinct size patterns for each glycoprotein, suggesting that they were distinct proteins. Most of the glycoproteins reacted with monoclonal antibodies which recognize a highly conserved epitope on rat P. carinii. Four of the individually purified glycoprotein preparations elicited in vitro cellular immune responses, implicating their involvement in the recognition of P. carinii by host T cells. The identification and characterization of P. carinii cell wall proteins will be helpful in analyzing the relationship of the organism to its mammalian host.

Detection of Pneumocystis carinii in induced sputa from immunocompromised patients using a repetitive DNA probe

A hybridization assay for the detection of Pneumocystis carinii was developed using a repetitive DNA fragment of P.c. hominis. The assay was specific as different micro-organisms typically found in the respiratory tract, normal human lung DNA (A 549 cell line) and normal rat lung DNA did not react with the repetitive probe. In a slot blot (SB) hybridization assay, the repetitive probe was able to detect as few as 100 P.c. hominis organisms with no false-positives. The results of the SB hybridization assay were compared with an immunofluorescence (IFA) assay for the detection of P.c. hominis in 84 induced sputum (IS) samples obtained from 52 human immunodeficiency virus (HIV)-seropositive patients, 22 HIV-seronegative patients and 10 healthy individuals. Samples from 24 patients clinically diagnosed with P. carinii pneumonia (PCP) were positive for P.c. hominis by both assays. In addition, the SB assay detected P.c. hominis in 14 patients (10 HIV-positive and four HIV-negative) who were negative by IFA. All 14 samples showed a positive PCR signal for the P.c. hominis dihydrofolate reductase gene, further confirming the presence of P.c. hominis in these specimens. Twelve of these patients had a clinical course highly suggestive of PCP and were either on P. carinii prophylaxis or P. carinii chemotherapy. The other two samples were from HIV-positive patients who had respiratory illness due to causes other than P.c. hominis (disseminated histoplasmosis and fatal Bordetella pneumonia). Detection of P.c. hominis in these samples suggests that these patients may have subclinical colonization by P.c. hominis. Furthermore, P.c. hominis was detected in all 12 sequential IS samples from six AIDS patients who had primary episodes of PCR using the SB assay, while P.c. hominis was detected only in eight samples by IFA (66.6%). All six patients developed recurrent PCP within 6 months from the time the assays were performed, further illustrating the potential of the SB hybridization assay in monitoring PCP recurrence. Thus, the ability of the SB hybridization assay to detect a low parasite load suggests that this assay may become an important supplemental tool, along with current cytological methods, for detecting P.c. hominis in patient populations with lower burdens of the organism and in identifying asymptomatic carriers of the parasite in healthy and immunosuppressed individuals.

Molecular characterization of a novel repetitive element from Pneumocystis carinii from rats

A repetitive DNA sequence, Rp-alpha, was isolated from rat-derived Pneumocystis carinii. The genome of rat-derived P. carinii contained 10 to 15 copies of Rp-alpha, which were located on most chromosomes, but no Rp-alpha could be detected in P. carinii derived from either humans or mice. Sequence analysis of two copies of the repeat showed them to be related but distinct. Each of them contained several copies of the 9-base sequence TAACCCTAA, arranged as direct repeats. Oligonucleotides consisting of multimers of this 9mer hybridized to the same set of chromosomes recognized by cloned copies of the Rp-alpha repeat. When used in DNA fingerprinting, the Rp-alpha repeat was capable of distinguishing between different isolates of rat-derived P. carinii.