Identification of an N-terminal 27 kDa fragment of Mycoplasma pneumoniae P116 protein as specific immunogen in M. pneumoniae infections.

Background: Mycoplasma pneumoniae is an important cause of respiratory tract infection and is increasingly being associated with other diseases such as asthma and extra-pulmonary complications. Considerable cross-reactivity is known to exist between the whole cell antigens used in the commercial serological testing assays. Identification of specific antigens is important to eliminate the risk of cross-reactions among different related organisms. Adherence of M. pneumoniae to human epithelial cells is mediated through a well defined apical organelle to which a number of proteins such as P1, P30, P116 and HMW1-3 have been localized, and are being investigated for adhesion, gliding and immunodiagnostic purposes.

Methods: A 609 bp fragment P116(N-27), corresponding to the N-terminal region of M. pneumoniae P116 gene was cloned and expressed. A C-terminal fragment P1(C-40), of P1 protein of M. pneumoniae was also expressed. Three IgM ELISA assays based on P116(N-27), P1(C-40) and (P116 (N-27) + P1(C-40)) proteins were optimized and a detailed analysis comparing the reactivity of these proteins with a commercial kit was carried out. Comparative statistical analysis of these assays was performed with the SPSS version 15.0.

Results: The expressed P116(N-27) protein was well recognized by the patient sera and was immunogenic in rabbit. P1(C-40) of M. pneumoniae was also immunogenic in rabbit. In comparison to the reference kit, which is reported to be 100% sensitive and 75% specific, ELISA assay based on purified P116(N-27), P1(C-40) and (P116(N-27) + P1(C-40)) proteins showed 90.3%, 87.1% and 96.8% sensitivity and 87.0%, 87.1% and 90.3% specificity respectively. The p value for all the three assays was found to be < 0.001, and there was a good correlation and association between them.

Conclusion: This study shows that an N-terminal fragment of P116 protein holds a promise for serodiagnosis of M. pneumoniae infection. The IgM ELISA assays based on the recombinant proteins seem to be suitable for the use in serodiagnosis of acute M. pneumoniae infections. The use of short recombinant fragments of P116 and P1 proteins as specific antigens may eliminate the risk of cross-reactions and help to develop a specific and sensitive immunodiagnostic assay for M. pneumoniae detection.