Decreased nematode clearance and anti-phosphorylcholine-specific IgM responses in mannose-binding lectin-deficient mice.

Brugia malayi is a nematode that causes human lymphatic filariasis. Previously, we showed that mannose-binding lectin (MBL)-A is necessary for clearance of B. malayi microfilariae in mice and presence of MBL-A is linked with maximal levels of parasite-specific IgM. Common human MBL gene polymorphisms result in low MBL expression and lead to recurring bacterial infections. Furthermore, these low-expressing human MBL polymorphisms result in greatly increased susceptibility to lymphatic filarial infection. Indeed, gain of new filarial infections over a 30-year period are 10-fold higher in people with low, compared to high, MBL-expression phenotypes. Human MBL closely resembles mouse MBL-C, rather than MBL-A; therefore, we examined the role of mouse MBL-C in clearance of microfilariae. Absence of MBL-C alone, or both MBL-A and -C, resulted in delayed clearance of microfilariae and reduced parasite-specific IgM in mice. There were few profound changes in B cell sub-populations or in the ability of MBL-deficient mice to respond to T-dependent or T-independent antigens. However, absence of MBL-A and/or MBL-C resulted in reduced IgM to phosphorylcholine, a constituent of filarial and bacterial antigens, suggesting that inability to form proficient antibody responses to this moiety leads to lack of microfilarial clearance and overall susceptibility to filariasis.

Mannose-binding lectin A-deficient mice have abrogated antigen-specific IgM responses and increased susceptibility to a nematode infection.

To investigate the role of mannose-binding lectin-A (MBL-A) in protection against infectious disease, MBL-A(-/-)-deficient mice were generated. Using a well-characterized mouse model of human filarial nematode infection, nematode survival and protective immune responses were tested in vivo. Blood-borne Brugia malayi microfilariae survived for significantly longer time periods in MBL-A(-/-) than in wild-type (WT) mice. However, no differences in either splenic cytokine responses or induction of leukocytes in the blood were observed. A profound abrogation of Ag-specific IgM levels was measured in B. malayi-infected MBL-A(-/-) mice, and some IgG isotypes were higher than those observed in WT animals. To establish whether there was a defect in Ab responses per se in MBL-A(-/-) mice or the effect was specific to filarial infection, we immunized these mice with OVA or a carbohydrate-free protein. Significantly, Ag-specific IgM responses were defective to both of these Ags, and Ag-specific IgG responses were largely unaffected. Furthermore, in naive mice, total IgM levels did not differ between MBL-A(-/-) and WT mice. This article describes the first demonstration that MBL-A may function independently of MBL-C and suggests that MBL-A, like other C-type lectins and members of the complement cascade, is intimately involved in the priming of the humoral Ab response.