Sequence polymorphisms within the pMGA genes and pMGA antigenic variants in Mycoplasma gallisepticum

Designing of novel chimeric PvpA-pMGA protein of Mycoplasma gallisepticum, applicable for indirect ELISA

Background

Mycoplasma gallisepticum is the primary agent of chronic respiratory disease in chickens creating important economic losses in poultry industry. pMGA and pvpA genes encode major surface proteins in M. gallisepticum containing pathogenic, antigenic, and immune evasion characteristics. The objective of the present study was to design, express, and purify the recombinant chimeric PvpA-pMGA protein from M.gallisepticum for using in serological diagnostic test.

Methods

Antigenic regions of PvpA and pMGA proteins were predicted for designing chimeric pvpA-pMGA gene construct. The codon optimized sequence was cloned into the expression vector pET32a+ and transformed into the Escherichia coli strain BL21 (DE3). The pET32a-PvpA-pMGA recombinant plasmid was expressed and confirmed by SDS-PAGE and immunoblotting. PvpA-pMGA recombinant protein (20μg and 50μg), ts-11 vaccine strain, and S6 strain that formulated by montanide adjuvant and two control groups (PBS and adjuvant) were injected subcutaneously to six groups of chickens.

Results

High yield of protein was purified amount 138 mg/L by affinity batch formation method. Indirect ELISA showed the levels of antibodies in rPvpA-pMGA was significantly higher than ts-11 and S6 groups (p<0.05). The results indicated that antigen-specific response was successfully elicited by the rpMGA-PvpA in chickens. The result of the ELISA with sera collected from ts-11 and S6 groups showed that indirect PvpA-pMGA-ELISA is appropriate candidate for detection of specific antibodies against M. gallisepticum with 100% sensitivity and specificity.

Conclusions

The rPvpA-pMGA is a highly candidate immunogenic protein which induced high amount of humoral immune response. Novel rPvpA-pMGA protein could be useful for evaluation of antibody level in vaccinated poultry flocks.

Application of high-resolution melting-curve analysis on pvpA gene for detection and classification of Mycoplasma gallisepticum strains

Mycoplasma gallisepticum (MG) is an avian species pathogen which causes heavy economic losses in the poultry industry. The purpose of this study was to determine genomic diversity of 14 MG field strains from chicken, Chuker partridge and peacock collected during 2009-2012 in Iran by polymerase chain reaction and partial sequencing of the pvpA gene. A High-Resolution Melting (HRM) technique was also developed and applied to differentiate between field and vaccine strains. Sequencing of the pvpA gene revealed a 51 nucleotide deletion, within DR-1 and DR-2, among MG strains from chicken and partridge whilst 63 nucleotides were deleted in MG strain from peacock. One nucleotide substitution was also observed among chicken MG strains. Phylogenetic analysis of the sequences clustered all of the Iranian MG strains into two clades or phylogeny groups; the strains from chicken and partridge in one group (group 1) and the strain from peacock into another group (group 4). HRM analysis has also produced comparable outcome to those of sequencing; four distinct melting curves which correspond to the three MG strains from chicken, Chukar partridge and peacock and ts-11 vaccine strain. Overall, findings of this study point towards a single source of infection for the chicken and partridge MG strains and likelihood of the strains being native and endemic in Iran. Peacock considered as an exotic species in Iran, hence the genetic distance for the pvpA gene. MG can be transmitted easily among different avian species and this distinct peacock strain may pose a threat to poultry industry. Our findings also show that molecular variation among pvpA gene of MG strains could be revealed using the relatively rapid and affordable HRM technique.

Cloning, expression, and antigenic characterization of recombinant protein of Mycoplasma gallisepticum expressed in Escherichia coli

Mycoplasma gallisepticum (MG) is a member of the most important avian mycoplasmas, causing chronic respiratory disease in chickens and leading to important economic losses in the poultry industry. Recombinant technology represents a strategic approach used to achieve highly reliable and specific diagnostic tests in veterinary diseases control: in particular this aspect is crucial for confirming mycoplasma infection and for maintaining mycoplasma-free breeder flocks. In this study, we identified a component of the pyruvate dehydrogenase dihydrolipoamide acetyltransferase (i.e., E2) protein by 2-dimensional electrophoresis (2-DE), characterized it in immunoblotting assays, and analyzed its recombinant (r-E2) in a rec-ELISA test. For full-length protein expression in Escherichia coli (EC) a point mutation was introduced. A rabbit antiserum produced against r-E2 was tested in a Western Blot using different samples of Mycoplasma species. The results showed the applicability of site-directed mutagenesis, with a good yield of the r-E2 after purification. Also, anti-E2 serum reacted with all the tested MG strains showing no cross reaction with other mycoplasmas. The developed E2 ELISA test was capable of detecting MG antibodies in the sera examined. Those results demonstrate the antigenic stability of the E2 protein which could represent a recombinant antigen with potential diagnostic applications.

Mycoplasma gallisepticum and Mycoplasma synoviae express a cysteine protease CysP, which can cleave chicken IgG into Fab and Fc

Major poultry pathogens M. gallisepticum and M. synoviae share a gene encoding a putative cysteine protease CysP similar to papain cysteine protease (C1A subfamily). Comparison of the cysP gene sequences of 18 M. synoviae and 10 M. gallisepticum strains sequenced in this study showed polymorphisms, including deletions. Seven M. synoviae strains, including the type strain WVU 1853, had a 39&emsp14;bp deletion in the 3' end of the cysP gene. In the same cysP region, all M. gallisepticum strains showed a deletion of 66&emsp14;bp. Immunoblot analysis with specific antibodies demonstrated that M. synoviae strains expressed CysP, which was approximately 65&emsp14;kDa. Both M. synoviae and M. gallisepticum were able to digest chicken IgG (cIgG). Incubation of cIgG (∼170&emsp14;kDa) with M. synoviae or M. gallisepticum cells (∼15&emsp14;h at 37&emsp14;°C) resulted in a papain-like cleavage pattern of cIgG and fragments corresponding to the antigen-binding fragment of IgG (Fab, ∼45&emsp14;kDa) and the crystallizable region fragment (Fc) of the IgG heavy chain (dimer of ∼60&emsp14;kDa). Iodoacetamide (50&emsp14;mM) prevented cleavage of cIgG by both Mycoplasma species. Following site-directed mutagenesis (eight TGA codons were changed to TGG) the cysP gene of M. synoviae ULB 925 was expressed as a His-tagged protein in a cell-free system. Purified recombinant CysP (rCysP; ∼67&emsp14;kDa, pI∼8) cleaved cIgG into Fab and Fc fragments. This indicates that CysP is responsible for the cIgG cleavage caused by M. synoviae and, probably, by M. gallisepticum. This is the first evidence to our knowledge that mycoplasmas have enzymes that can cleave the host IgG and indicates a novel strategy used by M. gallisepticum and M. synoviae for prolonged survival despite the antibody response of their host.

Mycoplasma gallisepticum Hemagglutinin VlhA, Pyruvate Dehydrogenase PdhA, Lactate Dehydrogenase, and Elongation Factor Tu Share Epitopes with Mycoplasma imitans Homologues

Mycoplasma gallisepticum is a major pathogen of poultry. Mycoplasma imitans is genetically and antigenically closely related to M. gallisepticum, but so far, only a few proteins of M. imitans have been identified as sharing epitopes with M. gallisepticum. In this study, we identified three proteins of M. gallisepticum that share with M. imitans epitopes defined by monoclonal antibodies (MAbs). MAb 9D4 reacted with the 67-kD hemagglutinin V1hA (previously termed pMGA) of M. gallisepticum and with its continuously expressed 40-kD protein. This MAb also reacted with a 40-kD protein of M. imitans, but not with its putative V1hA. Two-dimensional (2D) immunoblots of M. gallisepticum strains showed that their 40-kD proteins reacting with MAb 9D4 are expressed as major forms with isoelectric points (pI) around 6, and also as less-abundant forms differing in pI. In M. imitans, major forms of 40-kD proteins recognized by MAb 9D4 had pI around 6, whereas minor forms had pI between 5.5 and 5.8. The N-terminal sequence of the M. gallisepticum 40-kD protein recognized by MAb 9D4 strongly indicates that this protein is pyruvate dehydrogenase E1, subunit alpha (PdhA protein, also termed AcoA). The position of elongation factor Tu (EF-Tu), detected by the reference MAb GB8, was very similar in the 2D proteome maps of M. gallisepticum and M. imitans (MW of about 45 kD; pI - 5.6). In both M. gallisepticum and M. imitans, MAb 7G1 reacted with proteins of about 36 kD with similar charges (major forms with pI of about 8). The position of this protein in the proteome map of M. gallisepticum and its N-terminal sequence strongly suggest that MAb 7G1 recognizes lactate (malate) dehydrogenase (Ldh or Mdh). Comparison of 2D proteomes of 10 M. gallisepticum strains indicated that positions of EF-Tu, PdhA, and Ldh proteins are rather consistent and can be used as reference points in further analyses of the M. gallisepticum proteome.

Transfer of maternal immunoglobulins and antibodies toMycoplasma gallisepticumandMycoplasma synoviaeto the allantoic and amniotic fluid of chicken embryos

Maternal antibodies can protect avian embryos against vertically transmitted pathogens during embryogenesis and also young birds after hatching. In contrast to the well-known transfer of maternal immunoglobulin (Ig) G (also termed IgY) from the yolk to embryonic blood, information about the transfer of IgA, IgG and IgM from the egg albumen to the extra-embryonic fluids is very limited. In our study, IgA, IgG and IgM to Mycoplasma gallisepticum and Mycoplasma synoviae were detected in oviduct washings of naturally infected hens and in the corresponding egg albumen samples. In their progeny embryos, IgA, IgG and IgM antibodies to these Mycoplasma species were detected in the allantoic fluid (ALF) and amniotic fluid (AMF) on day 14 of embryonic development (ED). Examination of embryos from chickens immunized with antigens of M. synoviae revealed that the appearance of IgA and IgG and of antibodies to M. synoviae in ALF could vary even among embryos of the same dam. However, IgA, IgG and IgM were detected as early as day 7 of ED in ALF and AMF in certain embryos from hens infected with M. synoviae. In five groups of embryos examined on day 7, IgG to M.synoviae was found in 51% of ALF and 33% of AMF samples. M. synoviae was isolated from 2.3% of ALF samples. IgA to M. synoviae appeared in ALF and AMF on day 12 of ED, and could be found in the majority of AMF samples examined from day 14 onwards. IgM to M. synoviae appeared in AMF on day 13 and in ALF on day 14, but was detected in those fluids less frequently than IgA or IgG.

Characterisation of Mycoplasma gallisepticum strains involved in respiratory disease in pheasants and peafowl

Two cases of Mycoplasma gallisepticum infection in different avian species in backyard gamebird operations in Slovenia were investigated. In the first case, M gallisepticum was associated with severe respiratory disease with almost 20 per cent mortality in pheasants, whereas the infection was less pathogenic for chickens and turkeys reared at the same site. The M gallisepticum isolates from pheasants had a unique pMGA gene sequence containing a repeat of 12 nucleotides, and they contained only small amounts of the cytadhesins MGC1 and MGC3 and no PvpA protein. However, they expressed some typical M gallisepticum proteins and several proteins which were immunogenic for pheasants, chickens and turkeys. A strain of M gallisepticum isolated from the sinus of a pheasant was highly pathogenic for chicken embryos. In the second case, the M gallisepticum strain that was associated with respiratory disease and mortality in peafowl also affected chickens. M gallisepticum strain ULB 992 was isolated from the infraorbital sinus of a dead peafowl. The ULB 992 strain synthesised a small amount of MGC3, a truncated form of MGC1 and lacked PvpA. However, it expressed several proteins which were immunogenic for the birds infected with M gallisepticum at both gamebird operations.

Haemagglutinins of pathogenic avian mycoplasmas

The pathogenic avian mycoplasmas, Mycoplasma gallisepticum, Mycoplasma synoviae, Mycoplasma meleagridis, Mycoplasma iowae and Mycoplasma imitans, synthesize haemagglutinins that are immunogenic, variably expressed, surface proteins. The haemagglutinins of M. gallisepticum (pMGA), M. synoviae (VlhA) and M. imitans are lipoproteins, encoded by related multigene families that appear to have arisen by horizontal gene transfer. M. gallisepticum also has genes encoding cytadhesins in its genome but these are present as a single copies, while the pMGA gene family contains 30 to 70 genes. The switch in expression of distinct pMGA genes (e.g. pMGA1.1 to pMGA1.9) generates antigenic variation, which is thought to be important in immune evasion but also has significance in the preparation of M. gallisepticum antigens for serological diagnosis. In the majority of M. synoviae strains, post-translational cleavage of the VlhA protein generates an amino-terminal part (the lipoprotein MSPB) and a carboxyl-terminal part (MSPA), which mediates binding to erythrocytes. The 5'vlhA gene region, which encodes proline-rich repeats in the amino-terminal part of MSPB, is highly polymorphic among M. synoviae strains. Insertions or deletions in the part of vlhA encoding the proline-rich repeats cause MSPB length variation in different M. synoviae strains. Recombination between the 5'vlhA gene and pseudogenes in the genome generates changes in antigenic determinants in the carboxyl two-thirds of the MSPB molecule, and in MSPA, resulting in changes in the domains involved in the binding of M. synoviae to erythrocytes. Variant haemagglutinins of M. gallisepticum (pMGA1.7) and M. synoviae (diverse VlhA forms) share sequences that may be responsible for antigenic cross-reactions between M. gallisepticum and M. synoviae. Shared epitopes have been demonstrated using specific antibodies against MSPB that also recognize proteins of M. gallisepticum and of M. iowae (serotype N). Size and antigenic variants have also been reported for M. meleagridis and M. iowae proteins, but it is not known if these are their haemagglutinins. Advances in the molecular characterization of M. gallisepticum (pMGA, pvpA) and M. synoviae (vlhA) genes and their sequencing in numerous strains is likely to enable significantly improved epidemiological studies and improved tracing of M. gallisepticum and M. synoviae strains in different flocks.

Detection of antibodies to Mycoplasma gallisepticum vaccine ts-11 by an autologous pMGA enzyme-linked immunosorbent assay

Mycoplasma gallisepticum is a poultry pathogen that causes respiratory disease and loss of egg production worldwide. A live attenuated vaccine, ts-11, has been used for control of M. gallisepticum in several countries. The rapid serum agglutination test is usually used as an indicator of flock response to vaccination; however, in some flocks, the detected response may be weak or absent. With the use of specific monoclonal antibodies against M. gallisepticum strain S6 pMGA in immunoaffinity purification, the major membrane antigen of ts-11 was purified. An indirect enzyme-linked immunosorbent assay (ELISA) was developed with the purified antigen, and its potential for detection of antibodies induced after ts-11 vaccination was compared with an indirect ELISA with M. gallisepticum strain S6 pMGA. In the presence of high levels of ts-11-induced antibodies, both antigens detected similar numbers of positive sera. However, when lower levels of antibodies were present, ts-11 pMGA showed a higher sensitivity than S6 pMGA. Further examination of ts-11 pMGA with Mycoplasma synoviae-infected chicken sera revealed that ts-11 pMGA is specific for M. gallisepticum antibodies. With a panel of sera from ts-11-vaccinated or non-ts-11-vaccinated field chickens, the ts-11 pMGA ELISA was found to be more sensitive than the commercial rapid serum agglutination test in detecting antibodies to ts-11 vaccine. The results from this study suggest that the major membrane antigen of M. gallisepticum may have slightly different antigenic profiles in different strains, thereby necessitating the use of autologous antigens in serodiagnostic assays to increase sensitivity of the tests for mycoplasma antibodies. Thus, the low level of antibody response after ts-11 vaccination is, at least partially, due to the low ability of the current diagnostic antigens to bind ts-11 antibodies.

Molecular basis of the length variation in the N-terminal part of Mycoplasma synoviae hemagglutinin

Mycoplasma synoviae is a major avian pathogen that synthesizes hemagglutinin VlhA, an abundant immunodominant surface lipoprotein. In most M. synoviae strains, the VlhA protein cleaves into the N-terminal part, a lipoprotein MSPB, and a C-terminal part MSPA, which mediates binding to erythrocytes. VlhA is encoded by the vlhA gene of which the 5'-end is present in the genome as a single copy, which does not change its sequence during recombination of the vlhA gene with pseudogenes. In this study, sequence analyses of the 5'-end vlhA sequences of 30 M. synoviae isolates revealed a highly polymorphic region encoding the proline-rich repeats (PRR) in the N-terminal part of MSPB. Pathogenic strain K1968 had an insertion encoding sequence DNPQNPN in PRR, whereas strains F10-2AS, K2581, K3344 and five strains belonging to two related clusters of strains isolated recently from chickens in Slovenia lacked one PRR repeat of 19 amino acids. The predicted length variations correlated well with the lengths of the corresponding MSPB proteins detected in immunoblots with specific antibodies. Comparison of the 5'-end vlhA sequences of 30 M. synoviae strains showed 11 different types of vlhA sequences indicating that the analysis of this vlhA part is useful for strain differentiation. Distinct sequence motifs seem to be characteristic for vlhA genes of individual M. synoviae strains or clusters of strains and can be used as markers for tracing their spreading between poultry farms.