Interleukin-1α production during Rickettsia rickettsii infection of cultured endothelial cells: Potential role in autocrine cell stimulation

Abstract

Rickettsia rickettsii infection results in numerous responses by cultured endothelial cells, among them a rapid, transient increase in steady-state levels of tissue factor mRNA (L. A. Sporn, P. J. Haidaris, R.-J. Shi, Y. Nemerson, D. J. Silverman, and V. J. Marder, Blood 83:1527-1534, 1994). In this study, production of interleukin-I (IL-1) was measured during infection and its potential role in autocrine cell stimulation was investigated. A fivefold increase in levels of IL-1α antigen was measured in cell lysate samples by enzyme-linked immunosorbent assay at 18 h of infection. The majority of IL-1α remained cell associated, as no significant increase was detected in culture medium. No IL-1β antigen was detected in cell lysates or culture medium from either control or infected cultures. A dramatic increase in the levels of IL-1α mRNA occurred following infection, as measured by reverse transcriptase PCR, which revealed the appearance of the expected 421- kb product with RNA extracted from cells infected for 4 h and no detectable product from control cell samples. The presence of functional, cell- associated IL-1α, activity in infected cells was confirmed, following disruption, by the ability of the infected cells to induce tissue factor expression in target endothelial cells. Such induction was eliminated by pretreatment of the disrupted cell samples with neutralizing antibodies against IL-1α but not against IL-1β. To investigate whether endogenously produced IL-1 participates in the stimulation of tissue factor expression, neutralizing antibodies against IL-1 or the IL-1 receptor antagonist were added to culture medium during infection. Both anti-IL-1α and the IL-1 receptor antagonist resulted in an approximately 40% inhibition of tissue factor expression, thus implicating IL-1α in autocrine cell stimulation.