Elongated versions of Vlp surface lipoproteins protect Mycoplasma hyorhinis escape variants from growth-inhibiting host antibodies.

Variation in Vlp surface proteins of Mycoplasma hyorhinis was evaluated in terms of its role in determining susceptibility of organisms to growth inhibition by host antibodies (Abs). High-frequency switching of Vlp surface lipoproteins has been studied in isogenic lineages of M. hyorhinis SK76. In these lineages, the products of three genes, vlpA, vlpB, and vlpC, are subject to phase and size variation in vitro, which occur through distinct mutator elements that independently govern the expression of each vlp gene (promoter mutations) or the size of the vlp gene product (by intragenic expansion or contraction of a 3' region containing tandem repeats). Isogenic clonal variants of M. hyorhinis SK76 expressing distinct profiles of Vlp products were assessed for their susceptibility to complement-independent growth inhibition by serum Abs of swine experimentally infected with the arthritigenic SK76 strain. Invariably, variants expressing longer versions of VlpA, VlpB, or VlpC (each expressed individually) were completely resistant to host immune serum Abs, whereas variants expressing shorter allelic versions of each Vlp were susceptible. The target of growth-inhibiting Abs was not the Vlp products, since removal of anti-Vlp Abs had no effect on the inhibitory activity of the host immune serum on susceptible variants. Escape variant populations derived by propagating susceptible variants in an immune (versus control) host serum revealed a strong selection for the long-Vlp phenotype, irrespective of the identity of the Vlp expressed. Apparent mutational pathways of acquiring the protective phenotype included mutational switches to express long vlp genes that had been transcriptionally silent or switches to elongate expressed vlp genes. These results suggest that a major function of the Vlp system is to shield the wall-less mycoplasma surface from host Abs capable of binding vital (and as-yet-unidentified) surface antigens of this organism.