Mannose-Binding Lectin Gene Polymorphisms And Plasma Levels In Children With Recurrent Respiratory Tract Infection

Background: Mannose-binding lectin (MBL), a protein synthesized mainly in the liver, is a c-type lectin that plays an important part in innate immunity. It consists of a collagen-like region, a neck region, and a lectin domain that recognizes fucose, mannose, and N-actyl-glucosamine, which are present in a wide range of microorganisms. MBL protects the host from infection by lysis of microorganisms involving complement activation via the lectin pathway. In addition, MBL may enhance phagocytosis by direct opsonization involving MBL receptors on phagocytic cells.Plasma levels of MBL have previously been correlated with polymorphisms in the coding sequence and promoter regions. Mutations in codons 52, 54, and 57 of exon 1 in the MBL gene are believed to impair oligomerization and lead to functional MBL deficiency. Polymorphisms in the promoter region(at positions -550, -221, and +4, known as variants H/L, X/Y, and P/Q, respectively) influence the expression of this protein. Deficiency of this protein has been associated with an increased susceptibility to numerous infectious and autoimmune diseases. Recurrent respiratory tract infections(RRTI) are among the most frequent diseases in children. Except for those nutrition and enviroment factors, primary and acquired immunodeficiency disorders are considered to play an important role. MBL deficiency is considered to be the most common human immunodeficiency in many populations, so it may have some relationships with RRTI in children.Objective: To investigate the plasma MBL levels and gene polymorphisms of RRTI children in comparison with healthy controls, examine the humoral immune function status in RRTI chlidren, investigate the effect of combined MBL and immunoglobulin on the pathogenesis of RRTI.Methods: The concentrations of MBL in plasma were measured by ELISA kit. MBL promoter polymorphisms at nucleotides -550, -221, +4 and the exon1 polymorphisms at codons 52, 54, and 57 were analysed by PCR-SSP and PCR-RFLP in 70 RRTI children and 120 healthy controls. IgG, IgA, IgM, C3, C4 and IgG subclass levels in RRTI children were measured by immune nephelometry.Median plasma MBL concentrations among different groups were compared by Wilcoxon and Kruskal-Wallis tests. Immunoglobin and complement levels were compared by t test. Haplotype and genotype frequencies were compared byχ2 and Fisher's exact test (two-sided). P values <0.05 were considered statistically significant.Results:1. The median concentration of MBL in the sera of the patients was 1896 ng/ml(with a range of 0~5199 ng/ml); Compared with the reference value 161~5070ng/ml(P2.5~P97.5,n=470) we established before, the MBL plasma levels were significantly lower in RRTI children (p<0.05). 8 of the 70 RRTI patients had MBL levels less than 161 ng/ml.2. The A/A, A/B, B/B genotype at condon 54 polymorphism were correlated with high, low and undetectable MBL levlels(p<0.05).3. The H/H, H/L, L/L genotype at -550 polymorphism were correlated with high, medium and low MBL levlels (p<0.05). TheY/Y, Y/X, X/X genotype at -221 polymorphism were correlated with high, medium and low MBL levlels(p<0.05).There is no significant difference between the MBL levels of P/P, P/Q, Q/Q genotype at +4 polymorphism(p>0.05). Promoter genotype HY/HY, LY/LY, LX/LX were were correlated with high, medium and low MBL levlels (p<0.05). HYP carriers had significantly higher MBL levels than LXP carriers.4. The frerquency of B allele and genotype A/B was significantly higher in RRTI group. The LXP haplotype and LXP/LXP genotype were more common in RRTI group, the difference between the two groups was significant (p<0.05). In addition, the frequency of HYP haplotype was lower in RRTI group compared with healthy controls. The frequency of MBL insufficient genotypes (A/B+XA/XA+B/B) was much higher in RRTI group (p<0.05).5. IgG levels in RRTI chlidren were significantly lower compared with the controls (p<0.05), however, there is no significant difference of IgA, IgM, C3 levels. 20﹪RRTI children had IgG subclass deficiency, mianly IgG3-4. 3 of the 8 (37.5﹪) MBL deficient chiladren had combined IgG subclass deficiency.6.Whether combined with immunoglobulin deficiency or not, the frequency of MBL insufficient genotypes was significantly higher in RRTI group, and the plasma MBL levels were significantly lower compared with controls (p<0.05), whereas the distribution of MBL genotype between the two RRTI group was not significantly different (p>0.05).Conclusion:1. MBL polymorphims at codon 54 and -550, -221 in promoter appeared to significantly influence the plasma levels of MBL.The A, H, Y alleles were correlated with high MBL levels and the B, L, X alleles were correlated with low MBL levels. Promoter haplotype HYP was associated with the high MBL levels and LXP haplotype with low MBL levels. 2. Low MBL levels were associated with an increased susceptibility to RRTI in children. It is indicated B allele and LXP haplotype are risk factors of RRTI in children, nevertheless, high-producing haplotype HYP has some protective effect on RRTI.3. Low IgG levels and IgG subclass deficiency are important causes of RRTI in children.4. Whether combined with immunoglobulin deficiency or not, MBL dificiency is a risk factor of RRTI in chlidren. It is suggested that as a central player in innate immune system, MBL plays an important role to defense against infection through its"ante-antibody"function.