Objective  To study the effect of Listeria monocytogenes (LM) infection on dendritic cell (DC) immune function, and to compare the difference of immune activation ability on DC cell between bacterial form (WT) and L-form.

  Methods  C57BL/6 mice were randomly divided into 3 groups and infected intravenously with LM WT, LM L-form or PBS respectively. The ultrastructural characteristics of splenic DC were observed by transmission electron microscopy (TEM). The number of splenic DC, the expression of costimulatory molecules, the secretion of cytokine and the activation of splenic T lymphocyte were detected by flow cytometry.

  Results  TEM observation found that there were a large number of filamentous pseudopodia on the surface of splenic DC cells. The cytoplasm of DC was homogeneous and its nucleus was large. After phagocytosis of bacteria, the number of pseudopodia on the surface decreased and vacuoles in the cytoplasm increased. The number of splenic DC did not show significant changes at 1 d post infection (P>0.05). However, the expression of mature phenotypic molecules were significantly up-regulated (P < 0.05), in L-form infection group, both CD80 and CD86 molecules expressed on DC surface were higher than WT group (P < 0.05). Compared with control group, the proportion of TNF-α⁺DC were elevated after LM infection, and the average percentage of TNF-α⁺DC of L-form infection group was significantly higher than that of WT group (P < 0.05). At 7 d after infection, the average percentage of CD69⁺ T cells of L-form group was significantly higher than that of WT group (P < 0.05).

  Conclusion  LM L-form can induce relatively high levels of TNF-α and promote DC maturation so that to enhance their antigen presenting ability.