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Keywords:

  • immune;
  • infection susceptibility;
  • mannose-binding lectin;
  • polymorphism;
  • selection

Summary

  1. Top of page
  2. Summary
  3. From where has the high frequency of mannose-binding lectin polymorphism arisen?
  4. Different forms of evidence consider possible immune selection acting on MBL2
  5. Infectious diseases associations with MBL and possible affects favouring MBL2 heterozygotes
  6. New evidence regarding MBL and its effects in ischaemia reperfusion injury and predisposition to atherosclerosis suggest another selective pressure exerting an evolutionary influence on MBL2 gene frequency
  7. Conclusion
  8. Disclosure
  9. References

Either immune selection or stochastic processes may have influenced the frequency of highly polymorphic genes such as mannose-binding lectin 2 (MBL2). This pattern recognition receptor of the innate immune system recognizes and binds to pathogenic microorganisms and apoptotic cells leading to lectin pathway complement killing or clearance. In almost all of a large number of studies in different ethnic groups worldwide there is 20–25% carriage of low MBL2 haplotypes, with 8–10% of each population having no MBL detectable in the blood. The source of this high variability of MBL2 remains cryptic. It arises from six main snps in the prompter and exon regions of the gene that assort into seven common haplotypes under linkage disequilibrium. While global studies of MBL2 show that it is not under immune selection pressure, these results are not the same when the same population genetic tools are used on large national studies. Other analyses point to the silenced MBL1 pseudogene and development of promoter polymorphisms in humans as evidence of selection pressure favouring low-producing haplotypes. While these analyses cannot be reconciled readily, there are two processes by which MBL heterozygosity could have been advantageous in an evolutionary sense; protection against adverse effects of various infectious diseases and lethal manifestations of atherosclerosis – a disease that now seems to have a more ancient history than assumed previously. Ultimately, consideration of the context for possible future therapeutic manipulation of MBL means that this can proceed independently of resolution of the evolutionary forces that have shaped MBL2 polymorphism.


From where has the high frequency of mannose-binding lectin polymorphism arisen?

  1. Top of page
  2. Summary
  3. From where has the high frequency of mannose-binding lectin polymorphism arisen?
  4. Different forms of evidence consider possible immune selection acting on MBL2
  5. Infectious diseases associations with MBL and possible affects favouring MBL2 heterozygotes
  6. New evidence regarding MBL and its effects in ischaemia reperfusion injury and predisposition to atherosclerosis suggest another selective pressure exerting an evolutionary influence on MBL2 gene frequency
  7. Conclusion
  8. Disclosure
  9. References

The diverse role of mannose-binding lectin (MBL) in innate immunity, particularly its effects on infectious disease susceptibility, may indicate that an evolutionary advantage has led to selection for the high-frequency heterozygous states of MBL2. (Figure 1) This concept has been questioned, with some studies showing that MBL not only has no selection pressure acting on it but also with the authors concluding that therefore its immune action is redundant [1]. The very fact of MBLs pluripotency, that it does not just act in immunity to a single pathogen, means that there is almost certainly no balanced polymorphism driving MBL2 diversity, as is the case with haemoglobinopathies such as sickle cell anaemia or the presence of the red blood cell Duffy antigen and resistance to the malarias. Does this mean, however, that infection has not played a role in maintaining the very high frequency of polymorphisms seen in MBL2? With the multiple actions of MBL, including recognition of pathogen-associated molecular patterns, apoptotic cell clearance as well as lectin complement pathway activation in general, there may be multiple, concurrent effects of this molecule that have enriched the frequency of heterozygous states during human evolution.

figure

Figure 1. In attempting to account for possible selection pressures that may have enriched the frequency of MBL2 variant haplotypes, the various disease processes that could be more or less likely with high or low mannose-binding lectin (MBL) levels are shown in this figure. Suggested influences on MBL2 are indicated.

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MBL is a pattern recognition receptor produced by the liver that initiates the lectin complement pathway [2]. The MBL2 gene is located at 10q21.1, and single nucleotide polymorphisms in the promoter and structural gene sequences segregate under linkage disequilibrium to produce seven common haplotypes of MBL. Differential binding to target cell surface molecular (sugar moieties or nucleic acids [3]) patterns confers the specificity of MBL binding to microorganisms or apoptotic cells and not viable host cells. After binding to microbes, mannose-binding lectin-specific proteases attach to MBL and promote C4 binding with consequent lectin pathway killing or opsonophagocytosis [4].

Serum MBL levels are either low, intermediate or normal, correlating with the presence of low- (O/O and XA/O), intermediate- (XA/XA and YA/O) or high-producing (YA/YA) MBL2 genotypes (with exon variant alleles designated collectively as O and the wild-type gene as A, and the most influential promoter variant alleles and the wild-type gene designated as X/Y and L/H, respectively) [5]. Overall, approximately 20–25% of all populations defined carry low MBL2 haplotypes and 8–10% have no MBL detectable in the blood. The functional significance of low MBL2 genotypes or absent MBL protein has not been finally determined in individuals who are otherwise healthy. MBL deficiency (variously defined) is much more likely to be of relevance in contributing to overall immunocompromise in people with other immune system impairments such as neutropenia, or after transplantation [6] or immaturity, as in neonates [7].

It has been clearly demonstrated that MBL binds to and mediates killing or phagocytosis of numerous pathogenic microbes [4]. Furthermore, numerous studies have shown that either low MBL or low-producing MBL2 polymorphisms are associated with predisposition to infectious diseases [5]. Thus, this may be taken as evidence of infectious diseases providing immune selection for MBL2 polymorphisms. Whether this is truly the case may be revealed by the population genetics of MBL2 and the reliability of the information on infectious (or other disease) selective pressures.

Different forms of evidence consider possible immune selection acting on MBL2

  1. Top of page
  2. Summary
  3. From where has the high frequency of mannose-binding lectin polymorphism arisen?
  4. Different forms of evidence consider possible immune selection acting on MBL2
  5. Infectious diseases associations with MBL and possible affects favouring MBL2 heterozygotes
  6. New evidence regarding MBL and its effects in ischaemia reperfusion injury and predisposition to atherosclerosis suggest another selective pressure exerting an evolutionary influence on MBL2 gene frequency
  7. Conclusion
  8. Disclosure
  9. References

If MBL plays an essential role in protective immunity, then we would expect MBL2 deficiency alleles to be removed selectively at the population level, which is clearly not the case. If the presence of MBL is detrimental to some extent, then we would expect MBL2 deficiency haplotypes to be favoured and subject to positive selection, which might account for the high population frequencies of deficiency alleles.

Patterns of immune forces at work on selection of polymorphic proteins are described as purifying, balancing or neutral. According to population genetics theory, positive selection enriches the proportion of low-frequency alleles that provide a survival advantage. Where molecules under purifying selection are evolutionarily ancient, as in the case of MBL, which is found in organisms as primitive as marine sponges [8], their frequency will be optimized by natural selection. Balancing selection results in an excess of intermediate-frequency alleles (heterozygous advantage), such as the ongoing selective pressure on an individual's MHC exerted by exposure to novel pathogens. In conditions of simple genetic drift, a new mutation requires a long time to reach a high frequency in the population and results solely from human migration and genetic drift.

These different patterns of evolutionary pressure are determined by statistical tests of nucleotide variation within human populations and in comparison with intergenic species, the frequency of non-synonymous to synonymous mutation and the presence of linkage disequilibrium. When all these tools are applied to MBL in numerous studies that vary in the numbers and distributions of different ethnic groups included, quite different conclusions are reached as to the operant pattern of evolutionary pressure.

A global study of 1166 individuals from every geographic region concluded that while MBL showed three times the overall level of human genotypic diversity, the gene was not under selective immune pressure; rather, its frequency was drifting [1]. The FST (fixation index, a measure of population differentiation due to genetic structure) observed in this study was the same as that from other work [9] using a similar-sized sample from the Human Genome Diversity Panel–Center for the Study of Human Polymorphism (HGDP-CEPH) [10] and in the range of average genomewide FST analyses of genetic diversity. Smaller studies of various Brazilian native populations [11] and a comparison of Gabonese and European subjects have also shown no evidence of positive selection for MBL2 [12].

An analysis of MBL2 evolution in primates showed conservation of the gene across all 12 non-human primates studied in comparison with humans [1]. This interspecies conservation compared with human MBL2 diversity is suggested as further evidence of relaxation of ongoing selective pressure in humans after evolution from other hominids. Another analysis of the lack of primate interspecies variation suggested the importance of MBL in immune defences [13].

Other authors, using the same genetics tools but different populations, concluded that MBL2 has evolved under selective pressure. One study was restricted to North American, cosmopolitan, white populations, with this restricted genetic diversity potentially accounting for the different result [14]. Another study of Indian subjects showed that MBL2 haplotype frequencies differed significantly from global frequencies. These data are used to conclude that balancing selection does act upon MBL2, although tests for deviation from neutrality were neutral [15].

Using different genetic tools than these population analyses, other authors have suggested that MBL has been the subject of evolutionary selection [16]. Humans have only one functional MBL gene (MBL2), with MBL1 remaining only as a pseudogene after silencing mutations. This is taken, along with the evolution of human MBL2 promoter polymorphisms, which arose after separation from lower primates, to indicate that a selective advantage for low MBL levels led to these genetic events [16]. Further, the presence of putative disease associations and marked differences in the frequency and place-specific patterns of MBL polymorphisms arising from independent mutations [17] is promoted as evidence of positive selection pressure [18].

Population and other genetic studies do not give absolutely clear answers to the question of whether there is a balancing selection existing for MBL variant polymorphisms. We believe there are two main disease processes that could provide such selection pressure if it does exist. Interaction with infectious diseases, not necessarily a single pathogen and with complex effects of low and high MBL states, is the probable ‘driver’. However, as MBL is involved in atherosclerosis and ischaemia–reperfusion (I/R) injuries, commonly lethal, this process may also have a role in any selection pressure involving MBL.

Infectious diseases associations with MBL and possible affects favouring MBL2 heterozygotes

  1. Top of page
  2. Summary
  3. From where has the high frequency of mannose-binding lectin polymorphism arisen?
  4. Different forms of evidence consider possible immune selection acting on MBL2
  5. Infectious diseases associations with MBL and possible affects favouring MBL2 heterozygotes
  6. New evidence regarding MBL and its effects in ischaemia reperfusion injury and predisposition to atherosclerosis suggest another selective pressure exerting an evolutionary influence on MBL2 gene frequency
  7. Conclusion
  8. Disclosure
  9. References

The infection promoted most frequently as having provided an evolutionary advantage to individuals with MBL variant haplotypes has been tuberculosis [19]. This scourge, omnipresent throughout human history, co-evolved with our species. At first glance, tuberculosis (TB) would be an obvious force for selection of MBL polymorphisms because of its ubiquity and profound effects on reproductive fitness, with its extreme contribution to infantile mortality. Furthermore, it can be reasoned that low-producing MBL phenotypes may protect from TB by reducing phagocytosis of the organism, excluding it from its intracellular haven in macrophages.

However, this hypothesis has been investigated extensively, and various conflicting results returned indicating protection, predisposition or no influence on TB when MBL is analysed in various case–control studies in varying geographic locations. Comparison between studies in this area is hampered by heterogeneity in general and in particular by the overall lack of a unified definition of MBL deficiency and sufficiency. Our meta-analysis of 17 human studies considering the effect of MBL2 genotype and/or MBL levels and TB infection concluded that no significant association could be demonstrated between MBL2 genotype and pulmonary TB infection [20]. Unfortunately, however, the majority of studies did not report MBL2 haplotype inclusive of promoter polymorphisms, thus providing suboptimal evidence on which to base division of haplotypes into high- and low-producing groups. However, serum MBL levels were shown to be consistently elevated in the setting of TB, suggesting that the hypothesis of low MBL protecting against TB disease remains to be disproven, although this result may also be contributed to by an acute phase response elevating MBL.

An intriguing report of an association between the LYQC haplotype and protection from Mycobacterium africanum but not M. tuberculosis among Ghanaian patients has been published [21]. The binding of recombinant MBL was shown to be more efficient to M. africanum than M. tuberculosis. These data have not been reproduced in other African settings, but they indicate that the mycobacterial infection hypothesis providing evolutionary pressure for the high rates of MBL polymorphisms remains open.

Other candidate infections with high mortality and ancient history of human infections are Salmonella typhi and Trypanosoma. The S. typhi serovar evolved from its S. enterica parent 30–50 000 years ago and continues to cause high mortality, with an annual global burden of approximately 16 million cases and 600 000 deaths [22]. MBL binds to the lipopolysaccharide (LPS) of S. typhimurium and is consumed in Gram-negative sepsis [23]. No clinical studies of MBL polymorphisms and predisposition to typhoid fever have been performed. Just as with TB, it can be proposed that high MBL levels could predispose to typhoid, as it is also an intracellular pathogen. Similarly, intracellular parasitism such as visceral leishmaniasis [24] seems to be promoted by high levels of MBL.

Adults in Benin were found to have an exclusive enrichment of MBL2 exon 1 heterozygous mutations compared with newborns, suggesting a potential role of MBL in major infections and survival beyond infancy in Africa [25]. Hence, it may be that no single pathogenic micro-organism has been individually responsible for selection for MBL low-producing haplotypes. The group of intracellular pathogens described above may, together, have selected for individuals with low MBL levels. Conversely, however, it could be that the diarrhoeal illness and pneumonia that continues to contribute to 10% of all mortality is more frequent or more severe in patients, particularly children, with low levels of MBL increasing the progression of sepsis to shock [26]. This predisposition to severe infection may, in itself, be balanced by the consequences of sepsis that could be potentiated by the presence of high levels of MBL through complement-mediated tissue damage [27].

New evidence regarding MBL and its effects in ischaemia reperfusion injury and predisposition to atherosclerosis suggest another selective pressure exerting an evolutionary influence on MBL2 gene frequency

  1. Top of page
  2. Summary
  3. From where has the high frequency of mannose-binding lectin polymorphism arisen?
  4. Different forms of evidence consider possible immune selection acting on MBL2
  5. Infectious diseases associations with MBL and possible affects favouring MBL2 heterozygotes
  6. New evidence regarding MBL and its effects in ischaemia reperfusion injury and predisposition to atherosclerosis suggest another selective pressure exerting an evolutionary influence on MBL2 gene frequency
  7. Conclusion
  8. Disclosure
  9. References

A fascinating report of the prevalence of atherosclerotic lesions associated with ischaemic heart disease in mummies from prewestern civilizations raises the possibility that non-infective events may be another source of balancing evolutionary pressure selecting for MBL2 variant polymorphisms. The great record of ancient medicine, the Ebers Papyrus of 1550 bce [28], describes the characteristic symptoms of ischaemic heart disease thus: ‘If thou examinst a man for illness in his cardia, and he has pains in his arms, in his breast and on one side of his cardia … it is death threatening him’. If ischaemic heart disease existed in Egyptian and other contemporaneous civilizations it is likely to have been present for far longer. These civilizations did not have the high prevalence of traditional risk factors for ischaemic heart disease such as diabetes and hyperlipidaemia that are so common in the West, and another factor such as MBL may have contributed in as yet unappreciated ways to ischaemic heart disease. This hypothesis must be viewed against the background that a high proportion of young individuals (aged as young as 20 years) have atherosclerosis, with 5% having severe disease [29].

While protection from infection provides an obvious evolutionary survival benefit, the study by Thompson et al. [30] highlights atherosclerosis as an alternative selection factor. One hundred and thirty-seven mummies from four geographical regions and different time-periods (ranging between 1750 before the common era and 1900 common era) were evaluated for arterial calcification. Evidence of atherosclerosis was detected in 34% of the mummies across all populations, with an estimated mean age of death of 36 years. The presence of atherosclerotic disease in mummified ancient remains had been found previously, but a systemic study of a large number of bodies to provide an estimate of frequency of this disease had not been performed.

It seems plausible that non-traditional risk factors such as chronic infection and inflammation have promoted the development of atheromatous lesions in these individuals. Consequently, gene variants associated with enhanced immunity might have detrimental effects with regard to atherosclerosis. This might have influenced natural selection, even after reproductive age, as the likelihood of survival of half-orphans might have been severely compromised in ancient times. This influence operates currently, as a recent study found a markedly increased risk of death in young children in a 12-month period around the death of the mother (at a mean age of 35 years) in rural South Africa [31].

Atherosclerosis risk in ancient times was probably enriched by processes such as chronic infection and inflammation, rather than traditional risk factors such as diabetes and hyperlipidaemia. Contemporary individuals with rheumatoid arthritis (RA) or systemic lupus erythematosus (SLE) diseases, characterized by chronic inflammation and systemic activation of the immune system, might therefore be appropriate patients to compare with the younger victims of atherosclerosis with regard to the role of MBL in coronary vascular disease pathogenesis. RA and SLE are associated with excess mortality related partially to accelerated atherosclerosis and subsequent cardiovascular events [32, 33]. While traditional risk factors contribute to the increased risk for cardiovascular diseases in RA and SLE, they only partially explain the severity and progression of atherosclerosis in affected patients [34, 35]. Therefore, it is thought that excess production of inflammatory mediators of the immune system contributes significantly to the severity of atherosclerosis in RA and SLE patients [36]. In RA, high MBL serum levels are associated with a significantly increased risk of myocardial infarction and cardiovascular mortality [37, 38]. Interestingly, a U-shaped relation between serum MBL levels and intima-media thickness of the common carotid artery (ccIMT) was observed in RA patients independently of traditional and RA-related risk factors for atherosclerosis [39]. Middle-range MBL levels (corresponding to heterozygous genotypes) were associated with the smallest ccIMT compared with low and high levels. In SLE, evidence is less consistent. While MBL deficiency was associated with an increased ccIMT [40] and increased rate of cardiovascular events in earlier studies [41], larger cohort studies have not confirmed this association [42].

Apart from atherosclerosis in inflammatory diseases, MBL deficiency has been shown to represent a significant advantage in the setting of an acute cardiovascular event. MBL crucially augments I/R injury in animals [43] and humans [44, 45]. Several lines of evidence from murine models emphasize a central and active role for MBL in aggravating tissue damage after I/R injury of the heart [46], and other organs direct tissue damage and activation of the complement and coagulation cascade. In these studies, MBL knock-out mice consistently showed diminished deposition of complement proteins and neutrophil influx as well as limited tissue damage with favourable functional outcomes. In line with this, three recent observational studies identified MBL deficiency as a favourable prognostic factor after acute myocardial infarction [44] and ischaemic stroke [45, 47] in humans.

In summary, MBL deficiency might have been a significant advantage in terms of developing atherosclerosis in the setting of chronic inflammation and in particular during an acute cardiovascular event in ancient times. This could represent a possible selective factor even after reproductive age and might explain the remarkable frequency of heterozygous MBL2 mutations. While homozygous mutations might be clearly beneficial in terms of an acute ischaemic event, they certainly predispose to certain serious infections. Hence, MBL deficiency might be another example of a heterozygous advantage, as it might confer protection from I/R injury and atherosclerosis while exposing the individual to only a mildly increased risk to suffer from certain infections.

Conclusion

  1. Top of page
  2. Summary
  3. From where has the high frequency of mannose-binding lectin polymorphism arisen?
  4. Different forms of evidence consider possible immune selection acting on MBL2
  5. Infectious diseases associations with MBL and possible affects favouring MBL2 heterozygotes
  6. New evidence regarding MBL and its effects in ischaemia reperfusion injury and predisposition to atherosclerosis suggest another selective pressure exerting an evolutionary influence on MBL2 gene frequency
  7. Conclusion
  8. Disclosure
  9. References

If MBL2 polymorphism is neutral and the molecules immune effect redundant, does that mean it is not a useful therapeutic target? Whether the variation in MBL2 is due to immune selection or founder effects is of secondary importance to its potential role in immunotherapeutics. Arguments that are promulgated to suggest that MBL is redundant in the human immune system do not apply to the specific situations where this pattern recognition receptor could be used for treatment. Both patients with impaired phagocytosis due to chemotherapy-induced neutropenia and neonates with sepsis may be groups that could benefit from MBL supplementation. The requirement to search for optimized therapy for these specific patients is a product of the rapid evolution of developed world medicine, not the pressure of millennia of selection advantage. Preservation of myocardium or brain after reversal of ischaemic events with reperfusion may also be achieved by blocking the effects of MBL in the local circulation. While we now appreciate that atherosclerosis is not only a disease of modernity, the chance to intervene regularly in this critical illness is the preserve of the modern world and was denied the Pharaohs, despite their fabulous riches.

Human evolution has led to high levels of MBL2 polymorphism through neutral founder effects or positive selection. The high levels of conservation of MBL across primates indicate its importance as a pattern recognition receptor of the innate immune system. It is our contention that argument about the evolutionary forces that have enriched MBL2 variation should not distract from its possible uses as novel therapy for infectious diseases and ischaemia–reperfusion injury. Modern disease processes may provide specific niches for treatment approaches targeting this most evolutionarily ancient immune molecule.

References

  1. Top of page
  2. Summary
  3. From where has the high frequency of mannose-binding lectin polymorphism arisen?
  4. Different forms of evidence consider possible immune selection acting on MBL2
  5. Infectious diseases associations with MBL and possible affects favouring MBL2 heterozygotes
  6. New evidence regarding MBL and its effects in ischaemia reperfusion injury and predisposition to atherosclerosis suggest another selective pressure exerting an evolutionary influence on MBL2 gene frequency
  7. Conclusion
  8. Disclosure
  9. References