Characterization of the pattern of inflammatory cell influx and cytokine production during the murine host response to Listeria monocytogenes .

Abstract

To examine the physiologic mechanisms responsible for enhanced antibacterial activity during infection with Listeria monocytogenes (LM), we developed an in vitro assay for quantifying leukocyte anti-listerial activity (LAA) in spleen and bone marrow. When LAA was serially measured in C57B1/6 (B6) mice infected i.v. with LM, two distinct phases of response were observed. Splenic LAA increased four- to fivefold during the first 2 days after i.v. infusion of LM (from 2.4 +/- 1.8 U/spleen before infection to 11.8 +/- 2.4, p less than 0.01), dropped significantly on days 3 to 4, and increased again to similar levels from days 5 to 7. A fall in bone marrow activity from the 3.5 +/- 1.5 to 1.6 +/- 0.7 U/mouse (two femurs) coincided with the initial rise in splenocyte activity, and was followed by a gradual return to base line. Bacterial containment in vivo correlated well with splenic LAA in vitro. Carbonyl iron pretreatment of cells from both normal and LM-infected animals ablated LAA, suggesting the effectors were phagocytic. LAA in normal spleens was unaffected by 400 rad; LAA of normal marrow as well as splenocyte and marrow cell suspensions obtained 2 days after LM infection was markedly reduced by this dose of irradiation. Quantitative studies of spleen composition revealed a 10-fold increase in polymorphonuclear neutrophils between day 0 and day 2 followed by a marked decrease on day 3; this pattern closely resembled the changes in LAA observed during the same period. In contrast, splenic macrophage number did not increase from base line until after day 3. To look for evidence of changes in the efficiency of bacterial killing by phagocytes during infection, we calculated LAA/splenic phagocyte. The efficiency of killing increased threefold over base line within 1 day after LM infusion but we detected no additional increases later in infection. Because cytokines may have mediated some or all of the changes observed, we measured the capacity of splenocytes obtained at various times after infection to produce IL-2, TNF, and IFN-gamma in vitro. TNF activity increased in parallel with the first and second LAA peaks, whereas increases in IL-2 and IFN-gamma activity were associated only with the second. Although the importance of macrophages and the T cell lymphokines IL-2, and IFN-γ in the eventual control of LM infection has been appreciated for some time, the current studies suggest that 1) polymorphonuclear neutrophils are the dominant antibacterial effectors during the early "nonspecific" phase of listeriosis, and 2) TNF (perhaps in conjunction with other monokines) may be important in regulating phagocytic activity during this time period.