Antigenic mimicry of a human cellular polypeptide by Mycoplasma hyorhinis

Unveiling the stealthy tactics: mycoplasma's immune evasion strategies

Mycoplasmas, the smallest known self-replicating organisms, possess a simple structure, lack a cell wall, and have limited metabolic pathways. They are responsible for causing acute or chronic infections in humans and animals, with a significant number of species exhibiting pathogenicity. Although the innate and adaptive immune responses can effectively combat this pathogen, mycoplasmas are capable of persisting in the host, indicating that the immune system fails to eliminate them completely. Recent studies have shed light on the intricate and sophisticated defense mechanisms developed by mycoplasmas during their long-term co-evolution with the host. These evasion strategies encompass various tactics, including invasion, biofilm formation, and modulation of immune responses, such as inhibition of immune cell activity, suppression of immune cell function, and resistance against immune molecules. Additionally, antigen variation and molecular mimicry are also crucial immune evasion strategies. This review comprehensively summarizes the evasion mechanisms employed by mycoplasmas, providing valuable insights into the pathogenesis of mycoplasma infections.

Specific, high-affinity binding sites for angiotensin II on Mycoplasma hyorhinis

Mycoplasmataceae are known to express various proteins that are similar to those present in mammals. We report a strain of Mycoplasma hyorhinis isolated from opossum kidney cells with specific, high-affinity binding sites for human angiotensin II (Kd = 5.1 +/- 1.9 nM). In contrast, two strains of M. hominis revealed no specific binding. These binding sites resembled mammalian angiotensin II receptors by their high affinity and by their sensitivity to dithiothreitol. However, they are different from mammalian angiotensin II receptors in that they bind angiotensin I with high affinity (Kd = 1.6 +/- 0.29 nM) but not angiotensin III (Kd approximately 330,000 nM). [125I]-angiotensin II binding was not inhibited by angiotensin receptor subtype antagonists DuP 753 and CGP 42112A but it was sensitive to bacitracin and aprotinin. Positions Asp1, Ile5, His6 and Pro7 were essential for binding to M. hyorhinis as deletion of these residues led to a more than 10,000-fold decrease in affinity.

The potential role of mycoplasmas as autoantigens and immune complexes in chronic vascular pathogenesis

Many chronic degenerative diseases are suspected of resulting from autoimmune (AI) or immune complex (IC) mechanisms of unknown infectious etiology. Mycoplasmas have been associated with autoantibodies and tissue mimicry and with infertility and neonatal disorders, as well as with IC disorders. Their persistent colonization, which is clinically silent and common among healthy females, makes their etiopathogenic mechanisms difficult to assess. Finding the great apes to be colonized with and serologically expressing the human strains of mycoplasmas, as well as the anti-IgG rheumatoid factor (RF), provided us the ideal model of immunologically-related human disorders. In view of the etiopathologic role of IC in experimental vasculitis, especially collagen vascular diseases such as systemic lupus erythromatosus (SLE) with specific deposition on receptor sites of the vascular basement membranes, an investigation of IC's role in fetal and placental vascular disorders was initiated. The IgG-IC fractions isolated from solubilized human synovial and placental tissue extracts on affinity protein-A columns were further purified by pepsinization to minimize nonspecific Fc binding. Mycoplasmal antigens were identified in the F(ab)₂-IC fractions by reacting the electroblots from SDS-PAGE with rabbit antisera to 6 IgG test serovars. Finding mycoplasmal and ureaplasmal antigens associated with the IgG-IC in synovial and placental tissues suggests their pathogenic role in collagen vascular and reproductive disorders of both humans and the great apes. Mycoplasma cells cultured in rabbit-serum-enriched broth bound rabbit IgG as indicated by their specific reaction with goat anti-rabbit IgG. Polyclonal rabbit antisera raised against the mycoplasma cells binding the gnotobiotic rabbit IgG expressed idiotypic antibodies reactive with heterologous mycoplasmas cultured with whole sera but not with gamma-free or fetal bovine sera. Immunoblot analysis of SDS-PAGE fractionated mycoplasmas with the anti-M. pneumoniae rabbit IgG recognized common IgG epitopes in heterologous mycoplasma cells cultures with whole horse serum. These results demonstrate a potential role and pathogenic mechanism of mycoplasmas acting as a ligand, binding and causing conformational changes of IgG that could also apply to other specific tissue proteins. Conformational changes of bound tissue components could produce "altered-self" antigens to initiate a variety of autoimmune disorders in both humans and the great apes.

Recognition of binding sites on Candida albicans by monoclonal antibodies to human leukocyte antigens

Candida albicans exhibits examples of human molecular mimicry, including receptors resembling human steroid receptors and human complement receptor (CR)-like receptors that bind the complement fragments C3d and iC3b. To further characterize the epitopes of the Candida human CR-like molecules, a panel of monoclonal antibodies (MAbs) against epitopes within the human CR3 was used. MAbs Mo1, M1/70HL, and 7C3 bound to the germ tube, as assessed by immunofluorescence. MAbs Leu15, 60.1, and 95G8 did not bind. Miscellaneous MAbs against other antigens on human leukocytes (B2, 3D9, and OKT4) did not bind. However, MY9, which recognizes a monocyte antigen, bound extensively to the germ tube. The binding of certain anti-CR3 MAbs suggested limited identity between the Candida CR3-like receptor and the human CR3. The binding of MY9 identified an antigen recognized by a MAb to a human cell antigen not previously known to exist on C. albicans.

An approach to searching protein sequences for superfamily relationships or chance similarities relevant to the molecular mimicry hypothesis: application to the basic proteins of myelin

A rapid method for similarity searches (FASTP program) was used to identify similarities between a protein database and the human basic proteins from myelin [P2 protein and 17.2K, 18.5K, and 21.5K variants of myelin basic protein (MBP)]. From similarity scores, we concluded that none of the presently known proteins are in a family containing the MBPs. No new members were found for the lipid-binding family of which P2 is a member. Sequence similarities deemed relevant to the molecular mimicry hypothesis for virus-induced autoimmunity were identified in FASTP data with the aid of microcomputer programs. Several MBP/viral protein similarities were found that have not been reported previously. Of note because of their association with demyelinating conditions were proteins from visna and vaccinia. Similarity with visna was specific to the 21.5K and 20.2K MBPs. The most interesting new possibility for mimicry involving the P2 protein was between the influenza A NS2 protein and a sequence region of P2 thought to be neuritogenic in animals and mitogenic for lymphocytes from some patients with Guillain-Barré syndrome (GBS). This may have relevance for some cases of GBS associated with the 1976 U.S.A. swine flu vaccination program. Because FASTP reports only the best similarities between proteins, searches with FASTP may not have detected all the examples of mimicry present in the database. Searches might also be more effective if similarities could be scored on immunological rather than structural relatedness.