Epitope Mapping of a Protective Monoclonal Antibody against Pneumocystis carinii with Shared Reactivity to Streptococcus pneumoniae Surface Antigen PspA

Abstract

Pneumocystis carinii is an opportunistic fungal pathogen that causes pneumonia in the immunocompromised host. A protective monoclonal antibody (MAb) termed 4F11 generated against mouse-derived P. carinii was shown by indirect immunofluorescence assay (IFA) to bind surface antigens of P. carinii derived from multiple host species, including humans. We have identified multiple epitopes recognized by MAb 4F11 in two recombinant mouse P. carinii antigens. The epitopes mapped have similar proline content and positive charge distribution. The consensus 8-mer epitope recognized by MAb 4F11 is K/RPA/RPK/QPA/TP. Immune sera raised against intact mouse P. carinii recognized native antigens affinity purified with MAb 4F11 and a recombinant antigen reactive with MAb 4F11. Database searches for short, nearly exact matches to the mapped MAb 4F11 epitopes identified a bacterial surface antigen, Streptococcus pneumoniae PspA, with a similar proline-rich region. In an IFA, MAb 4F11 detected antigens on the S. pneumoniae surface, and Western blotting identified a protein in S. pneumoniae lysates consistent with the M r of PspA. A fragment of the S. pneumoniae PspA gene was cloned and sequenced, and the deduced amino acid sequence contained a region with strong similarity to the MAb 4F11 epitopes identified in P. carinii. The PspA recombinant polypeptide was recognized by MAb 4F11 in a Western blot. The ability of MAb 4F11 to recognize similar proline-rich epitopes may explain its ability to recognize P. carinii derived from multiple hosts and will permit testing of the epitopes recognized by this antibody in immunization against P. carinii.Pneumocystis carinii is an opportunistic fungal pathogen that causes pneumonia (P. carinii pneumonia [PCP]) in the immunocompromised host. PCP, as well as other opportunistic infections, underwent a dramatic rise in prevalence with the onset of the AIDS epidemic (28). With the development of highly effective antiretroviral therapy, the prevalence of PCP in AIDS patients has declined, though it remains the most commonly diagnosed serious opportunistic infection in AIDS patients (33). PCP is also prevalent in persons undergoing chemotherapy or other immunosuppressive therapy for cancer and organ transplantations (28). The most common drug treatments for P. carinii infections are trimethoprim-sulfamethoxazole and aerosolized pentamidine. Because adverse side effects, recurrent infections, and poor compliance are problems with these drugs, alternative treatments or preventative measures against PCP are needed to eradicate this serious opportunistic infection.P. carinii cannot be continuously cultured outside of its host. P. carinii also has a host species-dependent specificity which complicates the ability to use animal-derived organisms to immunize humans. P. carinii organisms derived from different hosts have isoform variants of common antigens, resulting in different (i.e., noncrossreactive) antigenic determinants (11, 13). Attempts to infect laboratory animals with P. carinii isolated from hete...

Immunization with Pneumocystis Cross-Reactive Antigen 1 (Pca1) Protects Mice against Pneumocystis Pneumonia and Generates Antibody to Pneumocystis jirovecii

“…A recent publication also describes boosting naturally acquired antibody in Pneumocystisexposed animals with a fragment of kexin as a means to enhance immune response to Pneumocystis (28). The presence of shared antibody epitopes between kexin and Pca1 warrants further analysis of vaccine immunogenicity and efficacy (17). The complete clearance of Pneumocystis from Pca1-immunized and -exposed animals reinforces the role Pca1 may have in PcP protection.…”

Complement and Fc Function Are Required for Optimal Antibody Prophylaxis againstPneumocystis cariniiPneumonia

“…In C. albicans, complement fixation has been linked to epitope specificity, location, and density (4,5). MAb 4F11(G1) recognizes multiple copies of similar epitopes that are uniformly distributed on the surface of the organism (13, 29), an attribute that likely contributes to protection by this antibody.…”

“…An IgG1 switch variant of IgM monoclonal antibody 4F11(G1) specific for mouse P. carinii Kex1 and cDNA clone A12 (2,13,29) and an anti-Haemophilus influenzae IgG1 monoclonal antibody were purified from ascites fluid by passage over a protein A-Sepharose column (Pierce, Rockford, Ill.).…”

“…MAb 4F11(G1) recognizes multiple, similar epitopes on the surface of P. carinii isolated from mice within at least two different antigens, kexin and cDNA clone A12 (13,29). MAb 4F11(G1) is also capable of recognizing P. carinii derived from humans, rhesus macaques, rats, and ferrets (2,29).…”

“…Our previous studies demonstrated that SCID mice are also protected from the development of PCP by passive prophylaxis using the anti-P. carinii immunoglobulin M (IgM) monoclonal antibody (MAb) 4F11 or its IgG1 switch variant, MAb 4F11(G1) (2). MAb 4F11(G1) recognizes multiple, similar epitopes on the surface of P. carinii isolated from mice within at least two different antigens, kexin and cDNA clone A12 (13,29). MAb 4F11(G1) is also capable of recognizing P. carinii derived from humans, rhesus macaques, rats, and ferrets (2,29).…”