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MHC class II genotype‐by‐pathogen genotype interaction for infection prevalence in a natural rodent‐Borrelia system

Abstract MHC genes are extraordinarily polymorphic in most taxa. Host‐pathogen coevolution driven by negative frequency‐dependent selection (NFDS) is one of the main hypotheses for the maintenance of such immunogenetic variation. Here, we test a critical but rarely tested assumption of this hypothesis—that MHC alleles affect resistance/susceptibility to a pathogen in a strain‐specific way, that is, there is a host genotype‐by‐pathogen genotype interaction. In a field study of bank voles naturally infected with the tick‐transmitted bacterium Borrelia afzelii, we tested for MHC class II (DQB) genotype‐by‐B. afzelii strain interactions for infection prevalence between 10 DQB alleles and seven strains. One allele (DQB*37) showed an interaction, such that voles carrying DQB*37 had higher prevalence of two strains and lower prevalence of one strain than individuals without the allele. These findings were corroborated by analyses of strain composition of infections, which revealed an effect of DQB*37 in the form of lower β diversity among infections in voles carrying the allele. Taken together, these results provide rare support at the molecular genetic level for a key assumption of models of antagonistic coevolution through NFDS.

and invertebrate host-pathogen systems. In contrast, in vertebrate host-pathogen systems, evidence for H G ×P G is as yet limited.
In vertebrates, much of the research on pathogen-mediated balancing selection has focused on MHC genes, which encode cell surface proteins playing a key role in the vertebrate immune system by presenting antigens to T lymphocytes (Radwan et al. 2020). NFDS has long been considered a potentially important driver of MHC polymorphism (Bodmer 1972;Hedrick and Thomson 1983). Yet to date, only a couple of studies have provided evidence for MHC genotype-by-pathogen genotype interactions, focusing on MHC class I (which encode MHC molecules expressed on all nucleated cells, presenting peptide antigens from proteins generated within the cell, including, e.g., viral proteins, to cytotoxic T cells). First, a study of virus adaptation through serial passage in inbred laboratory mouse strains congenic for different MHC class I haplotypes revealed trade-offs in viral fitness and virulence between mouse strains, thus providing proof-of-concept for H G ×P G involving MHC (Kubinak et al. 2012). Second, a field study of human malaria showed that the prevalence of different Plasmodium falciparum genotypes differed between individuals with and without a certain MHC class I allele in a P. falciparum genotype-specific way (Gilbert et al. 1998). Besides MHC, there are also a few recent human studies that have found that allelic variation at various non-MHC genes leads to H G ×P G (Ansari et al. 2017;Lees et al. 2019;McHenry et al. 2020;Band et al. 2021). However, H G ×P G at the molecular genetic level has to the best of our knowledge never been demonstrated in a natural nonhuman vertebrate host-pathogen system.
Here, we use a wild rodent, the bank vole (Myodes glareolus), and one of its natural pathogens, the tick-transmitted bacterium Borrelia afzelii, to test for MHC genotype-by-pathogen genotype interactions. As B. afzelii is primarily an extracellular pathogen, we focused on MHC class II (which encodes MHC molecules expressed on professional antigen presenting cells-i.e., dendritic cells, macrophages, and B lymphocytespresenting endocytosed antigens to T helper cells). One challenge when testing for H G ×P G is that it is difficult to know which pathogen gene might be involved. We circumvent this problem by testing for H G ×P G in the form of MHC class II genotype-by-B. afzelii strain interactions (Fig. 1), taking advantage of the fact that local populations of B. afzelii have a highly clonal genetic structure so that different strains can be distinguished by genotyping a single locus (here ospC) (Hellgren et al. 2011).

STUDY SYSTEM
The bank vole is a small, sexually monomorphic rodent and one of the most common and widely distributed mammals in Europe (Wilson et al. 2017). At the study site, bank voles mainly reproduce from May to September. Based on recapture data, individuals born early in the reproductive season reproduce the same season and rarely survive to the following year, whereas individuals born late defer reproduction to the following season (L. Råberg, unpubl. data).
The MHC class II molecule is a dimer composed of an α and β chain. Most mammals have three loci with classical MHC II genes (DQ, DR, and DP; although in, for example, Mus musculus, DP genes are pseudogenes), with each locus containing one or more gene copies encoding α chains (DQA, etc.) and one or more gene copies encoding β chains (DQB, etc.) (Kumánovics et al. 2003). Genes encoding β chains are generally more polymorphic than genes encoding α chains (Murphy and Weaver 2017). Moreover, previous analyses of bank vole MHC class II showed a larger number of positively selected sites in DQB than DRB genes (Scherman et al. 2014). In the present study, we therefore focused on DQB. Bank voles in the present study population can have up to eight DQB alleles, hence at least four DQB gene copies (Scherman et al. 2014).
Borrelia afzelii is a tick-transmitted bacterium and one of the causative agents of Lyme disease in humans (Kurtenbach et al. 2006). The natural hosts of B. afzelii are mainly rodents and shrews (Kurtenbach et al. 2002;Hellgren et al. 2011). At our study site, the bank vole is the most abundant host species (Råberg et al. 2017). Borrelia afzelii infections in bank voles are often chronic (Gern et al. 1994), but some individuals apparently clear infection, as indicated by loss of infection over winter (Scherman 2015). The overall prevalence of B. afzelii in bank voles at Kalvs mosse during May-October is 23.4% (Råberg et al. 2017). Prevalence increases with age (as determined by body mass), from a few percent in juveniles (<15 g) to around 30% in adults (≥20 g) Tschirren et al. 2013), presumably because older individuals have had longer time to acquire infection. Local populations of B. afzelii have a highly clonal genetic structure with virtually perfect associations between alleles at different loci, also between chromosomal and plasmid loci (Hellgren et al. 2011). Thus, different strains of B. afzelii can be distinguished by genotyping a single locus. We distinguished different strains by sequence genotyping of ospC, a plasmid-borne single-copy gene that is the most polymorphic locus in the Borrelia genome (Haven et al. 2011). ospC encodes the outer surface protein C (OspC), which induces a protective antibody response in the host (Jacquet et al. 2015). Thus, ospC could potentially be the focal gene involved in an H G ×P G with DQB, but a DQB × ospC could equally well be due to another B. afzelii gene encoding an antigen that is in linkage disequilibrium with ospC, and we stress that we here use ospC as a marker of B. afzelii strain identity. A given community of B. afzelii hosts (e.g., in a small forest) harbors five to 19 ospC strains (Hellgren et al. 2011;Durand et al. 2017). There are no differences in strain frequencies between host species (Råberg et al. 2017) and strain frequencies at a given site are stable over time for at least a decade (Durand et al. 2017;Råberg et al. 2017). Individual hosts are often infected with more than one ospC strain simultaneously (Strandh and Råberg 2015).

FIELD SAMPLING
Animals for this study were trapped at Kalvs mosse, a damp deciduous wood (ca. 24 ha) at Revingehed in southern Sweden (55°42ʹN, 13°29ʹE), in May-October 2006-2014. Animals were trapped with live traps (Ugglan special, GrahnAB, Sweden) baited with grains and apple or carrot. From each individual, we took a skin biopsy (∅ 2 mm) from the ear for DNA extraction. We also recorded body mass to the nearest 0.1 g using a Pesola spring balance. During 2007-2008, we collected longitudinal data by trapping bank voles about every 6 weeks during May-October and tagging each individual with a transponder (Trovan ID-100 Unique) subcutaneously implanted on the back, which allowed identification of each individual upon recapture.

DQB AND ospC SEQUENCING
Skin biopsies were stored in 70% ethanol until DNA extraction using the protocol of Laird et al. (1991).
Borrelia afzelii-infected bank voles were identified by realtime PCR of flaB (Råberg 2012). To determine which ospC strains an infected vole carried, we used 454 amplicon pyrosequencing, as described in Strandh and Råberg (2015). The ospC dataset used in the present study is the same as in Råberg et al. (2017).
Borrelia afzelii-infected voles were genotyped at DQB by amplifying 205 out of 272 bp in exon 2 using the primers MyglDQBfw and MyglDQBrv (Scherman et al. 2014), followed by amplicon sequencing. Exon 2 contains the majority of the peptide binding residues and is the most polymorphic exon in this gene (Scherman et al. 2014). Two different sequencing methods were used: 300 bp paired-end Illumina MiSeq sequencing (N = 265 in the final dataset) and 454 pyrosequencing (N = 36 in the final dataset; a subset of the data from Scherman et al. 2021). Libraries were prepared and the data filtered as described in Scherman et al. (2021). The concordance between Illumina MiSeq sequencing and 454 pyrosequencing of MHC is high (Razali et al. 2017). The reproducibility (number of DQB alleles detected in all replicates from an individual/total number of DQB alleles detected in that individual) was in all cases 100% for both Illumina MiSeq sequencing (samples from 34 longitudinally sampled voles; 2-4 samples/vole) and 454 pyrosequencing (13 technical duplicates; Scherman et al. 2021).

STATISTICAL ANALYSES
Analyses of repeatability of infection status with each ospC strain in infected bank voles were performed with rptR (Stoffel et al. 2017) in R 4.1.0 (RCoreTeam 2021), using rptBinary (separate analysis for each ospC strain). Link-scale approximation repeatabilities are reported.
Longitudinal analyses of number of ospC strains in infected bank voles were performed with proc mixed in SAS 9.4 (SAS Institute) with individual as random effect. Number of ospC strains was square-root transformed. We used general linear models with square-root-transformed data rather generalized linear models with Poisson distribution for the analysis of number of strains because the data were severely underdispersed.
Tests for effects of DQB × ospC on infection prevalence were performed as Generalized Linear Mixed Models (GLMM) with proc glimmix in SAS 9.4, with binomial error distribution. In all glimmix analyses, we used Laplace approximation, and assessed statistical significance of fixed and random effects by χ 2 and likelihood ratio (LR) tests, respectively, as recommended by Bolker et al. (2009). To examine the effects underlying a DQB × ospC interaction, we used the "slice" option in proc glimmix to test for effects of presence/absence of a particular DQB allele on prevalence of each ospC strain separately.
Analyses of effects of DQB genotype on ospC strain composition of infections were performed by PERMANOVA (Anderson 2001) and PERMDISP (Anderson 2006;Anderson et al. 2006), using the functions adonis2 and betadisper, respectively, in the vegan R package (Oksanen et al. 2020). PER-MANOVA primarily tests for differences between groups (here individuals with and without a specific DQB allele) in location in multivariate space, whereas PERMDISP tests for differences in dispersion (i.e., β diversity) between groups. Note, however, that results from PERMANOVA may be confounded by differences in dispersion between groups (Warton et al. 2012;Anderson and Walsh 2013). For all analyses of B. afzelii community composition, we used Euclidean distances based on presence/absence of ospC strains. We chose Euclidean distances, rather than, for example, Bray-Curtis distances that are more commonly used in analyses of microbial communities, because Euclidean distances take joint absences into account (Anderson et al. 2011). As we are interested in effects of host genotype on resistance/susceptibility to infection with particular ospC strains, joint absences are thus informative. To check the robustness of the results to the choice of distance metric, we also performed analyses based on Bray-Curtis (i.e., Sörensen, when analyzing presence/absence as done here) distances.
Date was coded as days since 1st January. Continuous variables (body mass and date) were Z transformed. Figures were generated with ggplot2 (Wickham 2016) and corrplot (Wei and Simko 2021).

Results
Data on ospC strain infection status were available from 395 B. afzelii-positive samples from bank voles collected during May-Oct 2006-2014 (same dataset as in Råberg et al. 2017). Fifty-six of these were recaptures, meaning we had ospC data from 339 individual bank voles. From 301 of these, we obtained data on DQB genotype; these 301 represent the dataset used for all analyses below (except longitudinal analyses).

BORRELIA INFECTIONS
Bank voles were infected with 11 different B. afzelii ospC strains. Four of these occurred in ≤2 bank voles each and were excluded from further analyses. As in previous analyses covering all host species (Råberg et al. 2017), the frequencies of the seven common ospC strains in bank voles did not vary among years (Fisher's exact test: P = 0.81). The number of ospC strains in an infected host individual ranged between one and seven (Fig. S1).
To test if infection status with each ospC strain was consistent over time, we used longitudinal data from recaptured voles (98 samples from 42 individuals; two samples from 30 individuals, three samples from 10 individuals, and four samples from 2 individuals; in all cases, individuals were resampled within a calendar year). For each of the seven common ospC strains, we estimated the repeatability of infection status. The repeatability (linkscale approximation) ranged from 0.29 (P = 0.02) for ospC2 to 0.94 (P < 0.001) for ospC1, showing that individuals were consistently infected or uninfected with each of the seven strains during a season.
To investigate how the number of strains in infected individuals varied over time, we performed a general linear mixed model with number of ospC strains (square-root transformed) against year, month (fixed factors), number of days after first sample of a given individual (covariate), and individual (random effect). The number of ospC strains varied among individuals (LR: χ 2 = 43.0, P < 0.0001) and increased over time (F 1,55 = 7.23, P = 0.009), but there were no effects of month, year, or their interaction (all P ≥ 0.11; Fig. S2). As the number of strains in an individual increased slightly over time, we used the last sample from recaptured individuals in the cross-sectional analyses below.

BANK VOLE DQB
The proportion of bank voles carrying each of the 48 DQB alleles observed in the study population is shown in Figure S3a. We selected alleles that occurred in ≥10% of bank voles for further analyses.

DQB × ospC
To test for ospC strain-specific effects of DQB alleles, we first performed GLMMs with infection status of a given ospC (coded as 0/1) against ospC strain (fixed effect, seven levels), presence/absence of a DQB allele (fixed effect), and ospC strain × DQB allele. To account for the nonindependence of infections with each ospC strain within each individual vole, we also included the factor individual as a random effect. We performed separate analyses for each of the 10 DQB alleles. In case of MyglDQB * 37, there was a significant MyglDQB * 37 × ospC interaction (P = 0.040; see Table S1 for full model details; Fig. 2a). In case of MyglDQB * 06, there was a significant main effect (P = 0.0054; see Table S2 for full model details; Fig. 2b). For all other alleles, both the main effect and interaction were nonsignificant (P ≥ 0.09 and P ≥ 0.22, respectively). The effects of MyglDQB * 37 × ospC and MyglDQB * 06 remained significant when including both terms in the same model (P = 0.041 and P = 0.0054, respectively). Examination of the MyglDQB * 37 × ospC interaction showed that MyglDQB * 37 had a positive effect on the prevalence of ospC7 and ospC10 (P = 0.0086 and P = 0.048, respectively), a negative effect on ospC1 prevalence (P = 0.045), but no effect on the prevalence of the remaining four ospC strains (P ≥ 0.46).

ospC STRAIN COMPOSITION
The conventional tests for DQB × ospC presented above were complemented with tests of effects of DQB alleles on multivariate ospC strain composition. Specifically, we analyzed two aspects of strain composition: mean abundance and variability (i.e., β diversity), corresponding to "location" and "dispersion" in multivariate space, respectively (Warton et al. 2012).
To test for effects of DQB alleles on abundance of specific ospC strains, we used PERMANOVA and performed separate tests with presence/absence of each of the 10 DQB alleles as factor. When using Euclidean distances, there were significant effects of MyglDQB * 37 (F 1,299 = 2.22, P = 0.033; Fig. 3a) and MyglDQB * 06 (F 1,299 = 2.22, P = 0.034; Fig. 3c; for all other alleles, P ≥ 0.30). When including both MyglDQB * 37 and * 06 in the same model, both remained significant (regardless of which order they were entered). When using Bray-Curtis distances, only the effect of MyglDQB * 37 was significant (F 1,299 = 3.15, P = 0.024; all other P ≥ 0.26).
To test for effects of DQB alleles on variability of ospC strain composition, we used PERMDISP and performed separate tests with presence/absence of each DQB allele as factor. When using Euclidean distances, there was a significant effect of MyglDQB * 37 (F 1,299 = 8.48, P = 0.0039; all other P ≥ 0.18; Fig. 3b,d). When using Bray-Curtis distances, there were effects of both MyglDQB * 37 and MyglDQB * 06 (P = 0.0090 and P = 0.0079, respectively).

Discussion
Here, we report the first direct test for MHC class II genotypeby-pathogen genotype interaction (H G ×P G ), a critical assumption behind the idea that polymorphism at MHC is a result of host-pathogen coevolution driven by NFDS. Using two different analytical approaches, we found evidence that allele MyglDQB * 37 has strain-specific effects on B. afzelii prevalence.
We acknowledge that at P = 0.04, the MyglDQB * 37 × ospC interaction does not reach significance when correcting for the number of genetic comparisons performed (a Bonferronicorrected α-value would be 0.05/10). However, an effect of MyglDQB * 37 was also found in the analysis of strain composition (PERMDISP, at P = 0.0039). Moreover, a previous study that compared B. afzelii-infected and uninfected bank voles (but without determining ospC composition of infections) showed that MyglDQB * 37 is associated with B. afzelii infection status. Specifically, voles carrying a haplotype including MyglDQB * 37 (the only haplotype with that allele) had higher prevalence of infection (Scherman et al. 2021), an observation that fits with the results of the present study where infected voles carrying MyglDQB * 37 had higher prevalence of the two most abundant ospC strains. Taken together, this indicates that the MyglDQB * 37 × ospC interaction observed in the present study represents a true biological pattern, rather than a type I error.
There are two basic types of models of antagonistic coevolution by NFDS: MA models and GFG models (Frank 1993;Agrawal and Lively 2002;Dybdahl et al. 2014). MA type models (including inverse MA) assume H G ×P G where host genotypes have opposite effects on resistance to different pathogen genotypes. GFG type models (including inverse GFG) instead assume that host genotypes differ in the range of pathogen genotypes they are resistant (or susceptible) to, such that one host genotype is resistant (or susceptible) to one or a few pathogen genotypes, whereas another host genotype is universally resistant (or susceptible) to all pathogen genotypes (Fig. 1). It is important to distinguish between these two types of models because MA models readily generate balanced polymorphisms, as resistance to one pathogen genotype comes with a cost in the form of susceptibility to another pathogen genotype. In contrast, the GFG scenario requires that resistance carries a cost in another currency to maintain polymorphism (Agrawal and Lively 2002).
Examination of the MyglDQB * 37 × ospC interaction showed it has elements of both MA and GFG type H G ×P G . Voles with MyglDQB * 37 had higher prevalence of ospC7 and ospC10 and lower prevalence of ospC1 than voles without MyglDQB * 37. Hence, MyglDQB * 37 appears to have opposite effects on resistance to ospC7/ospC10 and ospC1, consistent with the MA/inverse MA scenario (note that the lines for ospC7/ospC10 vs. ospC1 do not cross each other-like in the schematic representation of the MA/inverse MA scenarios in Fig. 1b,d-because ospC strains differ in abundance in the tick population, so that exposure rates differs; Råberg et al. 2017). This type of H G ×P G can maintain polymorphism on its own, as resistance to ospC1 comes at a cost in the form of susceptibility to ospC7 and ospC10. In contrast, a comparison of ospC1 or ospC7/ospC10 against the other four ospC strains (ospC2, ospC3, ospC4, and ospC9) reveals a pattern consistent with GFG/inverse GFG. For example, MyglDQB * 37 increased resistance to ospC1 but had no effect on resistance to ospC2, ospC3, ospC4, and ospC9. For this type of H G ×P G to maintain polymorphism, there needs to be a cost of the resistance allele in another currency that reduces the fitness of the resistance allele in the absence of infection (Agrawal and Lively 2002). In case of MHC, one potential type of cost is autoimmunity. Indeed, in humans, polymorphisms in the MHC region, in particular in MHC II genes, are associated with a number of autoimmune diseases (Karnes et al. 2017). The significance of autoimmunity in wild vertebrates, in particular short-lived ones like the bank vole, is virtually unknown, however.
To our knowledge, the only previous field study of a natural host-pathogen system that has explicitly tested for MHC genotype-by-pathogen genotype interactions is the study of human P. falciparum infection by Gilbert et al. (1998). The MHC class II genotype-by-B. afzelii genotype interaction observed in the present study is similar to the MHC class I genotype-by-P. falciparum genotype interaction found by Gilbert et al. (1998) in at least two ways. First, in both cases only one MHC allele was involved in an H G ×P G (one of 10 tested MHC class II alleles in the present study; one of 12 tested MHC class I and II alleles/haplotypes in Gilbert et al. 1998). Second, the strength of the effect of the interaction term is similar (P = 0.04 with N = 301 in the present study; P = 0.012 with N = 1207 in Gilbert et al. 1998) and probably in the range that can be expected for field studies that contain considerable noise due to chance exposure to different pathogen genotypes and other factors.
Our direct test for H G ×P G using GLMMs was complemented with analyses of effects of specific DQB alleles on ospC strain composition. These analyses corroborated our conclusion that MyglDQB * 37 has strain-specific effects on prevalence. The effect of MyglDQB * 37 on strain composition appears to be primarily driven by a difference in dispersion (β diversity) between voles with and without MyglDQB * 37, with strain composition of infections in voles carrying this allele being more homogenous than infections in voles without this allele. In contrast, allele MyglDQB * 06, which had an overall, rather than strainspecific, effect on infection prevalence in the GLMM, affected the multidimensional location, but not dispersion, of B. afzelii strain composition. Analyses of strain composition of infection, as used here, may be particularly valuable in cases where there are a large number of pathogen strains, and can complement conventional tests for H G ×P G when quantifying strain-specific resistance.
To conclude, our study is the first field study to provide evidence for H G ×P G in a nonhuman vertebrate host-pathogen system and shows that antagonistic coevolution driven by NFDS may contribute to the maintenance of the extraordinary polymorphism of MHC class II genes. The MHC genotype-by-ospC strain interaction observed in our study has elements of both GFG and MA type H G ×P G . Further analyses of other vertebrate hostpathogen system would be desirable to investigate how common H G ×P G are and whether they generally are of GFG or MA type.