Involvement of the lectin pathway of complement activation in antimicrobial immune defense during experimental septic peritonitis

Abstract

A critical first line of defense against infection is constituted by the binding of natural antibodies to microbial surfaces, activating the complement system via the classical complement activation pathway. In this function, the classical activation pathway is supported and amplified by two antibody-independent complement activation routes, i.e., the lectin pathway and the alternative pathway. We studied the contribution of the different complement activation pathways in the host defense against experimental polymicrobial peritonitis induced by cecal ligation and puncture by using mice deficient in either C1q or factors B and C2. The C1q-deficient mice lack the classical complement activation pathway. While infection-induced mortality of wild-type mice was 27%, mortality of C1q-deficient mice was increased to 60%. Mice with a deficiency of both factors B and C2 lack complement activation via the classical, the alternative, and the lectin pathways and exhibit a mortality of 92%, indicating a significant contribution of the lectin and alternative pathways of complement activation to survival. For 14 days after infection, mannan-binding lectin (MBL)-dependent activation of C4 was compromised. Serum MBL-A and MBL-C levels were significantly reduced for 1 week, possibly due to consumption. mRNA expression profiles did not lend support for either of the two MBL genes to respond as typical acute-phase genes. Our results demonstrate a long-lasting depletion of MBL-A and MBL-C from serum during microbial infection and underline the importance of both the lectin and the alternative pathways for antimicrobial immune defense.

FIG. 1.

Groups of wild-type mice (129/SV, n = 11), C1q-deficient mice (C1qa^−/−, n = 10), and factors B- and C2-deficient mice (H2-Bf/C2^−/−, n = 12) were subjected to CLP, and cumulative survival was monitored over 12 days. Compared to wild-type mice, the increase in mortality of C1qa^−/−mice was statistically insignificant (P = 0.2189), whereas the mortality of H2-Bf/C2^−/− mice was significantly higher (P = 0.001).

FIG. 2.

Bacterial counts were determined in spleen or liver cultures of groups (n = 5) of control 129/SV mice, C1qa^−/− mice, and H2-Bf/C2^−/− mice 24 h after CLP. Bacterial counts are given as CFU per organ. A statistically significant enhancement in bacterial counts from liver (P = 0.007) and spleen (P = 0.007) of H2-Bf/C2^−/− mice compared to C1qa^−/− was determined.

FIG. 3.

MBL-dependent C4 cleavage activity of mouse serum samples (NMRI; n = 5) at different time points after CLP and from untreated control mice was compared to an internal standard with the arbitrary activity of 1 U/ml. (A) Statistically significant reduced C4 activation in sera was determined at 10, 24, and 36 h after CLP. *, P < 0.05; **, P < 0.005. (B) MBL-dependent C4 cleavage activity 5 (n = 4), 7 (n = 4), 10 (n = 3), and 14 (n = 3) days after CLP. **, P < 0.005.

FIG. 4.

Serum levels of MBL-A and MBL-C were determined in mouse serum samples (NMRI) at different time points after CLP and compared to the serum levels of untreated control mice (n = 5). (A) MBL-A levels in untreated mice and 10 and 36 h after CLP. *, P < 0.05; **, P < 0.005. (B) MBL-A levels 5, 7, 10, and 14 days after CLP. (C) MBL-C levels in untreated mice and 10, 24, and 36 h after CLP. **, P < 0.005. (D) MBL-C levels 5, 7, 10, and 14 days after CLP.