Poor immunogenicity and vaccine failure in young children contribute to the ongoing burden of measles. The immunogenetic determinants of measles responses and vaccine efficacy in children are not yet well characterized. We aimed to identify whether genetic variation in the MV receptor gene CD46 contributed toward measles-specific antibody responses in an Australian cohort of infants immediately following their first contact with measles vaccine. We also aimed to investigate whether CD46 polymorphisms were associated with functional effects on the CD46 receptor in order to determine a possible mechanism through which the genetic variant acts on antibody responses. We demonstrated for the first time that CD46 polymorphisms are associated with cell-specific differences in receptor expression and with primary measles IgG antibody responses.
polymorphisms were associated with measles IgG levels (rs7144, rs11118580, and rs2724384), supporting the hypothesis that CD46
plays an important role in how a child responds to measles vaccine. Associations between CD46
and measles antibody levels have been identified in previously vaccinated American school-aged children and adults (7
). The associations with rs11118580 and rs2724384 in our naive infants agree with those found in the older children and adults (7
), with the same alleles (C and G, respectively) associated with the lower measles antibody levels. However, in contrast to the results of Dhiman et al. (7
), we have identified, for the first time, a significant association between CD46
rs7144 and measles antibody levels. Genetic differences may have a different impact upon the naive immature immune system of infants than they would have on older children and adults once their immune systems have matured. Our results showed that CD46
variants may be involved in determining primary measles antibody responses in naive young children.
genotypes were significantly associated with measles antibody levels; however, they did not seem to determine whether or not a child was protected against measles (however, with only 10% seronegative children in this cohort, there was insufficient power to assess this properly). Regardless, measles antibody levels are known to wane over time (20
), so children with a lower primary antibody response (even above the protective level) are more likely to fall below the protective level and become vulnerable to measles in a shorter period of time. These children may also produce lower levels of memory B cells (which would be interesting in future studies) which may affect their responses to their second dose of measles vaccine. Although our results showed associations with altered antibody levels in certain CD46
genotypes, this has unclear clinical relevance at this stage, and therefore further studies would need to determine whether lower antibody levels translate to poor immunogenicity and/or vaccine failure.
We have shown that genetic variation in the CD46
gene was associated with measles antibody responses, but it is not known what the mechanistic link is between these two variables. No known functional effects of CD46
polymorphisms have been published. We hypothesized that the CD46
SNPs that associate with measles IgG would functionally alter the expression of the CD46 receptor. We have reported, to our knowledge, the first data on the associations of a polymorphism in a measles receptor gene and receptor protein expression. A variant allele of a CD46
3′UTR polymorphism (rs7144) was associated with lower measles antibody levels in our Perth cohort. The 3′UTRs of genes can contribute to the regulation of mRNA translation and stability and thus protein expression (31
). This polymorphism therefore has the potential to alter the expression of CD46 on the cell surface, thereby modulating the amount of receptor able to bind to MV, leading to a decrease in the immune response against measles.
We found that the highest CD46 expression was in monocytes, which follows previous in vivo
studies that demonstrated that monocytes are the cell type predominantly infected by MV (10
). After interaction with MV hemagglutinin, CD46 is internalized and downregulated from the cell surface (4
). We found CD46 expression was downregulated after measles lysate (ML) stimulation of PBMCs, dramatically in B cells and monocytes, with a smaller change in CD4+
T cells. Expression in mDCs, however, increased 3-fold after ML stimulation. It appears that the T-cell expression of CD46 is not activation induced, but the expression in APCs is considerably altered after contact with measles.
CD46 expression in T cells was not overly induced by ML stimulation, but it was significantly associated with the CD46
rs7144 SNP. Children with the rs7144CC genotype had significantly more CD46 expression at baseline in T cells but not in APCs. There was also a distinct difference between the genotypes in the regulation of CD46 expression in T cells; those with the CC genotype showed higher downregulation of the receptor after ML stimulation. This downregulation may affect CD46 signaling and antigen presentation (30
), as well as increase the probability of cell lysis (15
). Children with rs7144CC also showed a decreased frequency of T cells after ML stimulation, perhaps reflecting this T-cell lysis.
Our results showing this CD46 rs7144 SNP associated with receptor expression and T-cell frequency suggest a possible mechanism that links genetic variation and altered measles antibody response (). Those with rs7144CC had a higher density of receptor on the T-cell surface, higher downregulation of receptor from the surface after measles (higher rate of MV binding and hence infection of the cell), and possible increased T-cell lysis (which is supported by our results that showed a decrease in T-cell frequency). This may lead to decreased T-cell help for measles-specific B cells and hence impaired measles antibody responses and, in fact, we showed that those with this same genotype had lower levels of measles IgG. It would be important to test this hypothesis of the mechanisms underlying how receptor expression affects antibody responses in additional functional studies.
Summary of results regarding CD46 rs7144 polymorphism
CD46 rs11118580 and rs2724384, although showing significant associations with measles IgG levels, did not show any relationships with CD46 protein expression. Unlike 3′UTR SNPs such as rs7144 and polymorphisms in the promoter region, the intronic CD46 rs11118580 and rs2724384 variants are likely to functionally affect CD46 protein in an alternative way, perhaps affecting structure, trafficking, or localization of the protein, rather than the amount of expression. It would be important to investigate the functionality of these SNPs in future studies.
The limitations of the present study should be noted. First, the size of the Perth cohort was small, and therefore the study was not powered to detect the effect of the SNPs on the proportion of seronegative children. However, our sample size is comparable to other published studies in this area (7
). Although it would be interesting to compare these initial responses with second dose responses, this cohort was not followed up at 4 years, and therefore comparisons could not be made. Corrections for multiple testing were not performed in the present study, since only a limited number of tests were performed, with the analyses based on specifically defined a priori
hypotheses, and all of the tests that were performed were described (21
). Although we showed a decrease in T-cell frequency following ML stimulation, we have not specifically quantified cell lysis; therefore, this is only a possible theory and should be followed up in future studies. It is possible these SNPs are in linkage disequilibrium with another functional polymorphism in CD46
or in another gene responsible for the association with measles antibody response. Alternative host genetic factors may also have an effect, such as genes of the HLA system (19
) and cytokine and cytokine receptor genes (6
) that have been shown to influence measles vaccine responses in adults. In addition, receptors such as the other measles-specific receptor, signaling lymphocyte activation molecule (SLAM), are likely to have effects on measles responses (7
). We have, however, recently shown that SLAM
polymorphisms are not associated with measles antibody responses in this cohort (3
). Regardless, it is biologically plausible and likely that CD46 plays an important role in specific responses to measles virus and vaccine.
In conclusion, we report significant associations with genetic variants in measles receptor CD46 and primary IgG antibody responses to measles vaccine in a population of Australian infants after their first vaccine dose. We also demonstrate the first findings of an association with an MV receptor polymorphism and functional effects on receptor protein expression, a possible explanation for the link between genetic variation and altered measles vaccine responses. In the future, the combination of genetic factors that reliably predict poor immunogenicity or vaccine failure need to be elucidated. Knowledge in this area may help in the identification of children at risk of poor immunogenicity and vaccine failure, which may lead to the development of different strategies for vaccinating these children and contribute to the control of MV infections worldwide.