Major outer membrane protein Omp25 of Brucella suis is involved in inhibition of tumor necrosis factor alpha production during infection of human macrophages

Brucella spp. can establish themselves and cause disease in humans and anim als. The mechanisms by which Brucella spp. evade the antibacterial defenses of their host, however, remain largely unknown. We have previously reporte d that live brucellae failed to induce tumor necrosis factor alpha (TNF-alp ha) production upon human macrophage infection. This inhibition is associat ed with a nonidentified protein that is released into culture medium. Outer membrane proteins (OMPs) of gram-negative bacteria have been shown to modu late macrophage functions, including cytokine production. Thus, we have ana lyzed the effects of two major OMPs (Omp25 and Omp31) of Brucella suis 1330 (wild-type [WT] B. suis) on TNF-alpha production. For this purpose, omp25 and omp31 null mutants of B. suis (Delta omp25 B. suis and Delta omp31 B. s uis, respectively) were constructed and analyzed for the ability to activat e human macrophages to secrete TNF-alpha. We showed that, in contrast to WT B. suis or Delta omp31 B. suis, Delta omp25 B. suis induced TNF-alpha prod uction when phagocytosed by human macrophages. The complementation of Delta omp25 B. suis with WT omp25 (Delta omp25-omp25 B. suis mutant) significant ly reversed this effect: Delta omp25-omp25 B. suis-infected macrophages sec reted significantly less TNF-alpha than did macrophages infected with the D elta omp25 B. suis mutant. Furthermore, pretreatment of WT B. suis with an anti-Omp25 monoclonal antibody directed against an epitope exposed at the s urface of the bacteria resulted in substancial TNF-alpha production during macrophage infection. These observations demonstrated that Omp25 of B. suis is involved in the negative regulation of TNF-alpha production upon infect ion of human macrophages.