Diabetes 54:3002-3006, 2005 © 2005 by the American Diabetes Association, Inc. Elevated Levels of Mannose-Binding Lectin at Clinical Manifestation of Type 1 Diabetes in Juveniles
1 Department of Surgery, Leiden University Medical Center, Leiden, the Netherlands
Mannose-binding lectin (MBL) is a recognition molecule of the lectin pathway of complement and a key component of innate immunity. MBL polymorphisms have been described that are associated with MBL serum concentration, impaired function, and diabetic complications. We investigated 86 new-onset juvenile type 1 diabetic patients and compared these with their nondiabetic siblings and healthy unrelated control subjects. Polymorphisms of MBL exon 1 and promoter were determined, and serum concentration and MBL-complex activity were measured. Although the genetic polymorphisms of MBL were not different between patients and control subjects, MBL serum concentration as well as MBL complex activity was significantly higher in new-onset diabetic patients compared with their siblings matched for high-producing MBL genotypes (P = 0.0018 and P = 0.0005, respectively). The increase in MBL complex activity in high-MBL–producing patients could only partially be explained by high MBL production, as demonstrated by an increased MBL complex activity–to–MBL concentration ratio (P = 0.004). We conclude that MBL serum concentration and complex activity are increased in early- onset diabetic patients upon manifestation independently of genetic predisposition to high MBL production, indicating a possible role in the immunopathogenesis of type 1 diabetes, in addition to the adaptive islet autoimmunity.
Address correspondence and reprint requests to Dr. Bart O. Roep, Department of Immunohaematology and Blood Transfusion, E3-Q, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, Netherlands. E-mail: boroep{at}lumc.nl
Abbreviations: CRP, C-reactive protein; mAb, monoclonal antibody; MASP, mannose-binding lectin-associated serine protease; MBL, mannose-binding lectin; SNP, single nucleotide polymorphism Type 1 (insulin dependent) diabetes is an autoimmune disease characterized by the specific destruction of ß-cells in the pancreas. The etiology of type 1 diabetes is multifactorial, consisting of genetic predisposition and environmental factors including a variety of viruses and dietary components (1,2). The role of the adaptive immune system in the autoimmune process leading to type 1 diabetes is well established. Presently, the interest for the innate immune system in the immunopathogenesis of type 1 diabetes is mounting (3–5). That the recognition of self-determinants is confined to the adaptive immune system is generally proposed, diminishing the role of the innate immune system in autoimmunity. However, evidence is growing that changes in the innate immune system could lead to autoimmunity, either by priming or promoting aggressive immune responses (6). Mannose-binding lectin (MBL) is a key molecule of the innate immune system and is able to bind common carbohydrate structures of a variety of microorganisms (including bacteria, viruses, and fungi), resulting in direct opsonophagocytosis and complement activation. In plasma, MBL is associated with MBL-associated serine proteases (MASPs). Upon binding of MBL to its ligand, the subsequent MASP-2 activation is responsible for complement activation via the lectin pathway (7). Exon 1 of the mbl-2 gene contains three known single nucleotide polymorphisms (SNPs) at codons 52 (referred to as allele "D"), 54 (allele "B"), and 57 (allele "C") (8). These SNPs are associated with low serum concentrations, disturbed polymerization, and impaired function of MBL (9,10). Dependent on ethnicity, the allele frequency of variant alleles B, C, and D, commonly referred to as O-alleles, may be above 40% (wild type = A/A). In addition to the three SNPs in exon 1, there are several other polymorphic sites located in the MBL promoter region, including SNPs located at positions –550 (H/l variant) and –221 (X/Y variant). The common allele A of exon 1 is associated with the following haplotypes: HYA, LYA, and LXA, exhibiting respectively high, intermediate, and low promoter activity and serum MBL levels. The structural alleles carry the following haplotypes: LYB, LYC, and HYD (11,12). Low MBL serum levels and genetic polymorphisms associated with impaired MBL function have been shown to be associated with different autoimmune diseases including celiac disease and systemic lupus erythematosis (13,14). Although the complement system has been studied in diabetes (15), the association between MBL and the immunopathogenesis of diabetes has not yet been investigated to any extent. MBL has been associated with vascular complications in diabetic patients. High-MBL genotypes are significantly more frequent in diabetic patients with nephropathy than in normoalbuminuric diabetic patients. Furthermore, comparing patients with identical MBL genotypes, serum MBL levels were higher in patients with nephropathy than those with normoalbuminuria (16,17). Recently, high MBL levels in the early course of type 1 diabetes were shown to be associated with development of albuminuria, indicating that MBL may be involved in the pathogenesis of diabetic microvascular complications (18). We decided to address the possible association between MBL and the pathogenesis of type 1 diabetes. We hypothesize that as the insulin production diminishes during insulitis, MBL serum concentration will rise as a consequence of the inflammation process. MBL in turn could promote the adaptive immune response, either via enhanced complement activation or increased opsonophagocytosis of autoantigens, interweaving MBL in the complex autoimmune process of type 1 diabetes. To test our hypothesis, we studied 86 juvenile type 1 diabetic patients at clinical presentation. With the intention to match for age, genetic background, municipality of residence, and other environmental factors, an unaffected sibling of every diabetic patient was included as a control subject. For genetic analysis, a healthy, unrelated control group was included in the study of 69 voluntary healthy blood donors. MBL genotype, concentration, and complex activity were further correlated with diagnostic and predictive parameters as serum fructosamine levels, the presence of islet autoantibodies, and HLA type.
Meeting all legal and ethical criteria set out by the local and ethical committees, fresh peripheral blood samples were obtained from 86 juvenile type 1 diabetic patients at diagnosis (mean ± SD, age 9.3 ± 3.5 years, 34 girls). Diabetes was diagnosed according to the criteria set out by the World Health Association (19). For every patient, a sibling control subject was included as control for serological assessment of MBL concentration and MBL complex activity (age 10.3 ± 4.8 years, 36 girls). To avoid a parental selection bias, a control group of 69 healthy blood donors was included for allele frequency analysis. Serum was immediately aliquoted and frozen at –70°C. DNA was routinely isolated from heparinized blood.
MBL genotyping. Promoter SNPs located at positions H/l (–550) and Y/X (–221) were typed by PCR using sequence-specific priming. The conditions for PCR amplification and primer sets that are used in this study are available in the online appendix (available at http://diabetes.diabetesjournals.org). For analysis, MBL genotypes HYA/HYA, HYA/LYA, LYA/LYA, HYA/LXA, and LYA/LXA were considered high-MBL–producing genotypes. Low-MBL–producing genotypes were defined as LXA/LXA, HYA/O, and LYA/O. Genotypes LXA/O and O/O were considered MBL deficient.
MBL concentration.
MBL complex activity.
HLA typing.
Autoantibody typing.
C-reactive protein concentration.
Fructosamine concentration.
Statistical analysis.
MBL genotype. The allele frequency of SNPs located in exon 1 and the promoter region of the mbl-2 gene were compared between patients and healthy unrelated control subjects. No significant difference in allele frequency of the exon 1 or promoter SNPs could be observed between patients and healthy control subjects. Full MBL genotype characterization of all patients and sibling control subjects are available in the online appendix.
MBL serum concentration.
MBL complex activity. Next to the MBL concentration, we also examined MBL function by measuring MBL complex activity. When we compared MBL complex activity between patients and sibling control subjects, MBL complex activity was strongly elevated in diabetic patients (P = 0.01, Mann-Whitney test; Fig. 2). Stratifying both patients and sibling control subjects according to high-MBL–producing, low-MBL–producing, and MBL-deficient genotypes revealed that MBL complex activity is strongly dependent on MBL genotype in both patients and in sibling control subjects (P < 0.0001, ANOVA). However, MBL complex activity was approximately twofold higher in diabetic patients with a high-producing MBL genotype than in the sibling control subjects (P < 0.00005, Mann-Whitney test; Fig. 1). No difference between patients and sibling control subjects was observed for the low-MBL–producing and MBL-deficient genotypes.
Comparing MBL concentration and MBL complex activity. The MBL concentration was related to the MBL complex activity in both diabetic patients and the sibling control subjects. MBL concentration was strongly correlated to MBL complex activity in both groups (P < 0.0001, rs = 0.87, Spearman test; table available in online appendix). Because the MBL complex activity shows a stronger elevation in patients compared with sibling control subjects than the MBL concentration (Fig. 1), we normalized the amount of MBL by calculating a ratio. The MBL complex activity–to–MBL concentration ratio was compared between patients and sibling control subjects in accordance with the MBL genotype. Patients with a high-MBL–producing genotype showed a significantly increased ratio (mean 1.6) compared with sibling control subjects (mean 1.1) (P = 0.004, Mann-Whitney test; Fig. 2A).
HLA, autoantibodies, and fructosamine concentration.
CRP.
Our study demonstrates that serum MBL levels and MBL complex activity are elevated at clinical manifestation in juvenile type 1 diabetic patients with high-MBL–producing genotype compared with sibling control subjects. The complex activity was higher within the group of high-MBL–producing genotypes of type 1 diabetic patients, suggesting that the increase was associated with the immunopathogenesis of type 1 diabetes, rather than genetic variation. Interestingly, the ratio between MBL concentration and MBL complex activity was also significantly higher in the high-MBL–producing patient group, signifying a greater activity per molecule MBL. This indicates that MBL function in new-onset diabetic patients is increased in addition to elevated MBL protein concentration. The increase in functional MBL activity in diabetic patients could be a result of immune hyperactivity. Although MBL has been suggested to act as an acute-phase protein (23), several studies have been unable to show an association between MBL and CRP as an acute-phase reactant (16,17,24). Our studies confirm the absence of an association between both MBL concentration and MBL complex activity and CRP. This renders acute-phase reactivity as an unlikely explanation for the increased MBL levels and activity in new-onset type 1 diabetes. Alternatively, it could be argued that the significant association of MBL complex activity, but not MBL serum concentration, with fructosamine serum levels implies that this is a consequence of poor glycemic control, rather than a surrogate of immune hyperactivity. Nonetheless, in view of a lack of association of MBL serum levels in either subpopulation of patients with fructosamine levels, we favor the interpretation that the increased serum levels are associated with the immunopathogenesis of type 1 diabetes, while the actual MBL complex activity is affected by glycemic control. Elevated glucose levels resulting in high MBL complex activity could facilitate the adaptive autoimmune response by means of direct opsonophagocytosis of aberrantly glycosylated autoantigens. Finally, it should be appreciated that in plasma, MBL function is dependent on its association with serine proteases (MASPs). Currently, there are three known MASPs, MASP-1, MASP-2, and MASP-3, all of which have a different function. Among these, MASP-2 is responsible for cleavage of C4 and C2 and generation of the C3 convertase C4bC2a (25). It could be hypothesized that an increase in the MBL complex activity on top of an increased MBL serum concentration is a result of preferential binding of MASP-2 to MBL, resulting in a higher C4 splicing ability. Furthermore, in addition to increased MBL serum concentration in high-MBL–producing genotypes, MASP-2 levels could be elevated and result in more prominent MBL complex activity. Finally, it could be hypothesized that the increase in MBL complex activity could be the result of reduced inhibition. Fluid-phase complement inhibitors like C1 esterase inhibitor have been shown to inhibit MASP activity (26). Impairment of complement inhibitors as a result of increasing hyperglycemia could clarify an increased complement activating capacity of MBL with poor glycemic control. The observation that serum concentration and complex activity were not increased in either the low-MBL–producing or MBL-deficient genotypes of MBL in type 1 diabetic patients confirmed our expectation that these genotypes are unable to facilitate a sufficient MBL response in type 1 diabetic patients and in nondiabetic control subjects. In concurrence with our conclusion, previous studies have shown a lack of association between MBL serum levels in diabetic patients and poor glycemic control (27). Interestingly, it has been suggested that an increase in MBL serum concentration as an acute-phase response can be suppressed by intensive insulin therapy, which fortifies our conclusion of the contribution of MBL in the pathogenesis of type 1 diabetes (24). A direct implication would be that low-MBL–producing and MBL-deficient MBL genotypes could have a beneficial effect on type 1 diabetes, because the onset may be less fulminant. In any case, low-MBL–producing genotypes and MBL-deficient genotypes are favorable for diabetic patients, in addition to a potential role of MBL in the pathogenesis, because high MBL serum levels have been shown to be associated with vascular complications (17). In conclusion, we suggest that elevated MBL levels, resulting in increased complement activation, could assist the autoimmune process of insulitis, pathognomonic for early stages of type 1 diabetes, and act as a marker for ongoing insulitis. This effect may be enhanced by an increased MBL complex activity as a result of poor glycemic control.
This study was supported by the Dutch Diabetes Research Foundation (grant 97.137), the Netherlands Organization for Health Research and Development, and the Juvenile Diabetes Research Foundation International (grant 2001.10.004).
Additional information for this article can be found in an online appendix at http://diabetes.diabetesjournals.org. Received for publication May 9, 2005 and accepted in revised form June 27, 2005
This article has been cited by other articles:
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||