A critical role of natural immunoglobulin M in immediate defense against systemic bacterial infection

Abstract

To evaluate the role of natural immunoglobulin (Ig)M in the immediate response against microbial infection, we tested mutant mice that are deficient in secreted (s)IgM in an acute peritonitis model induced by cecal ligation and puncture (CLP). 20% of wild-type mice died within 32 h of CLP, whereas 70% of sIgM-deficient mice died within the same time period. The increased susceptibility was associated with a reduced level of tumor necrosis factor (TNF)-alpha, a decreased neutrophil recruitment and an increased bacterial load in the peritoneum, and elevated levels of endotoxin and proinflammatory cytokines in the circulation. Resistance to CLP by sIgM-deficient mice was restored by reconstitution with polyclonal IgM from normal mouse serum. Reconstitution with a monoclonal IgM specific to phosphatidylcholine, a conserved cell membrane component, has a modest effect but a monoclonal IgM specific to phosphocholine is not protective. These findings demonstrate a critical role of natural IgM in the immediate defense against severe bacterial infection.

Figure 1

Natural IgM confers resistance to CLP. sIgM-deficient (−/−) and wild-type (+/+) mice at 6-8 wk of age were subject to CLP. IgM-reconstituted sIgM-deficient mice (−/− IgM) were given as a single dose of 0.5 mg i.v. of total IgM affinity- purified from normal mouse serum 4 h before CLP. Mice were monitored for survival within the first 32 h.

Figure 2

Comparison of the levels of LPS, TNF-α, and IL-6 in the serum of sIgM-deficient and wild-type mice at different time points after CLP. Sera were collected at 1.5, 3, 6, and 12 h after CLP and divided into four groups based on the genotype of the mice and whether the mice died or survived at 32 h after CLP. Sham control was operated on without ligation and puncture. Concentrations of LPS, TNF-α, and IL-6 were determined by ELISA. Error bars indicate SD. For TNF-α assay, the numbers of mice used in each category are as follows: −/− dead, 12; −/− survived, 10; +/+ dead, 10; and +/+ survived, 16. Eight mice were used in each category for IL-6 assay. Six mice were used in each category for LPS assay. Numbers of sham controls for TNF-α, IL-6, and LPS were 2, 1, and 1, respectively.