Mice lacking monocyte chemoattractant protein 1 have enhanced susceptibility to an interstitial polymicrobial infection due to impaired monocyte recruitment

Abstract

Monocyte chemoattractant protein 1 (MCP-1) is an important chemokine that induces monocyte recruitment in a number of different pathologies, including infection. To investigate the role of MCP-1 in protecting a host from a chronic interstitial polymicrobial infection, dental pulps of MCP-1(-/-) mice and controls were inoculated with six different oral pathogens. In this model the recruitment of leukocytes and the impact of a genetic deletion on the susceptibility to infection can be accurately assessed by measuring the progression of soft tissue necrosis and osteolytic lesion formation. The absence of MCP-1 significantly impaired the recruitment of monocytes, which at later time points was threefold higher in the wild-type mice than in MCP-1(-/-) mice (P < 0.05). The consequence was significantly enhanced rates of soft tissue necrosis and bone resorption (P < 0.05). We also determined that the MCP-1(-/-) mice were able to recruit polymorphonuclear leukocytes (PMNs) to a similar or greater extent as controls and to produce equivalent levels of Porphyromonas gingivalis-specific total immunoglobulin G (IgG) and IgG1. These results point to the importance of MCP-1 expression and monocyte recruitment in antibacterial defense and demonstrate that antibacterial defense is not due to an indirect effect on PMN recruitment or modulation of the adaptive immune response.

FIG. 1.

Infection results in enhanced tissue necrosis in MCP-1^−/− mice. Surgical pulp exposure followed by inoculation with six oral pathogens was carried out as described in Materials and Methods. Hematoxylin- and eosin-stained sections were examined for the presence of tissue necrosis in the dental pulp. (A) Lower half of the mesial root of the first molar. The arrows point to necrotic dental pulp in an MCP-1^−/− mouse and normal dental pulp in a wild-type (WT) animal. (B) Percentages of necrosis. The length of necrotic tissue was compared to the total length of the dental pulp to calculate the percentage of necrosis. The means ± standard errors of the means (n = 5 for each time point) are shown. Significant differences (P < 0.05) were noted between the experimental and wild-type mice at 14 and 21 days following bacterial challenge. (C) Percentages of mice exhibiting greater than 75% necrosis of the dental pulp calculated for days 0 to 1, 3 to 7, and 14 to 21.

FIG. 1.

Infection results in enhanced tissue necrosis in MCP-1^−/− mice. Surgical pulp exposure followed by inoculation with six oral pathogens was carried out as described in Materials and Methods. Hematoxylin- and eosin-stained sections were examined for the presence of tissue necrosis in the dental pulp. (A) Lower half of the mesial root of the first molar. The arrows point to necrotic dental pulp in an MCP-1^−/− mouse and normal dental pulp in a wild-type (WT) animal. (B) Percentages of necrosis. The length of necrotic tissue was compared to the total length of the dental pulp to calculate the percentage of necrosis. The means ± standard errors of the means (n = 5 for each time point) are shown. Significant differences (P < 0.05) were noted between the experimental and wild-type mice at 14 and 21 days following bacterial challenge. (C) Percentages of mice exhibiting greater than 75% necrosis of the dental pulp calculated for days 0 to 1, 3 to 7, and 14 to 21.

FIG. 2.

Monocyte recruitment and osteolytic lesion formation in MCP-1^−/− and wild-type (WT) mice. The dental pulp was challenged with six oral pathogens. Animals sacrificed at day 14 were immunostained with F4/80 antibody specific for peripheral monocytes and macrophages. The small arrows indicate immunostained cells. The large arrows indicate the space between the apex of the root and the surrounding bone. This space is enhanced as a result of osteolysis in the MCP-1^−/− mice.

FIG. 3.

MCP-1^−/− mice have reduced monocyte recruitment in response to infection. The dental pulp was challenged with six oral pathogens. Monocytes and macrophages were identified by immunostaining with the F4/80 antibody, and the numbers were determined by computer-assisted image analysis. (A) Total number of F4/80-positive cells per lesion. (B) Number of F4/80-positive cells per square millimeter in each lesion. The means ± standard errors based on five or six specimens for each time point are shown. WT, wild type.

FIG. 4.

MCP-1^−/− mice have enhanced osteolysis resulting from infection. The area of each lesion at the dental root apex was determined with a computer-assisted image analysis system at different times following exposure of the dental pulp and inoculation of bacteria. For each value the area at zero time representing the normal space between the root apex and bone was subtracted. The means ± standard errors based on five or six specimens are shown. WT, wild type.