In this article we have focused our analyses on allelic variants associated with phenotypic variability. Many genetic features contributing to MDD may have swept to fixation over evolutionary history and by becoming nonpolymorphic remain invisible to genetic association studies. It is possible that such sequences are preferentially associated with species-typical social, rather than immunological, factors. Were this to be the case, our analyses may have overestimated immune risk factors for depression at the cost of universal depressogenic risk alleles maintained as a ‘price of being human'.
It should also be noted that inflammatory biomarkers are not elevated in all individuals with MDD. Whether patients with increased inflammation represent a biologically and evolutionary distinct subset of MDD is an area of active research.388
If this turns out to be the case, it may be that selection for enhanced host pathogen defense is relevant primarily to these individuals (and their allelic variants) and is thus only one adaptive factor driving the persistence of depressogenic alleles. Prior theorists have posited a variety of potentially fitness-enhancing psychosocial effects of low mood and/or depression not obviously related to host defense functions (that is, abandoning unattainable goals, yielding in dominance struggles and so on),389, 390, 391
and it may be that these types of psychosocial benefits are promoted by allelic variants retained in the human genome independently of variants maintained as a result of conferring pathogen host defense benefits. If both immune and nonimmune etiological pathways contribute to MDD, the next question is how they combine. One hypothesis consistent with the general absence of documented epistatic interactions among MDD risk alleles is that inflammation/immune alleles provide one hit and social/stress factors provide a second (biologically distinct) hit, which together sum to exceed an MDD symptom threshold. If distinct social and immune-related genetic risk factors were identified, statistical analysis of epistasis could help distinguish intrinsic interactions between pathways from a simple additive model.
On the other hand, findings from patients undergoing treatment with IFN-α suggest a more inclusive scenario for the role of pathogen defense in the evolution/persistence of depressogenic alleles. Specifically, although standardized dosages of IFN-α are employed, a wide range of behavioral responses are observed during treatment, from mild neurovegetative/sickness symptoms, such as fatigue, to completed suicide in response to catastrophic major depression. Individuals who develop significant depressive symptoms evince changes in CNS and neuroendocrine functioning that are also observed in idiopathic MDD,305, 306, 307, 308, 309, 310
but that are not observed to a significant degree in patients on IFN-α who do not develop depression. These findings raise the possibility that depression reflects a state of immune response element amplification, such that for any given amount of inflammatory input, depressed individuals react with enhanced downstream CNS/neuroendocrine activity. If so, depressogenic alleles that do not promote an increase in inflammatory biomarkers may nonetheless have undergone positive selection because they enhanced host pathogen defense via sensitization of downstream CNS/neuroendocrine pathways that themselves promote survival during infection. Some evidence for this hypothesis comes from the finding, noted above, that individuals who develop depression during IFN-α manifest enhanced sickness behavior at the start of treatment, which may aid in acutely clearing pathogens from the body during infection.302
Moreover, a clear prediction of PATHOS-D theory is that changes in CNS/neuroendocrine function that typically accompany MDD should enhance survival in the context of acute infection. To date, few data support this possibility, although it is intriguing to note that glucocorticoid resistance, which is common in MDD392
and is associated with the development of depressive symptoms during IFN-α treatment (Raison et al.
, unpublished observations), has been associated with improved T-cell function in HIV infection,393
and that enrichment paradigms known to enhance glucocorticoid sensitivity in animal models increase mortality in response to Escherichia coli
These two possibilities (that is, distinct additive social and immunological risks vs inflammatory mediation of social risk) might be genetically discriminated based on their contrasting implications for the functional relationship between social–environmental precipitants and immune-related genetic risks for MDD. In the former model, one would expect to find largely additive effects of social risk factors and immune-related genetic risk alleles, whereas the meditational model would suggest a product-term interaction (that is, a social stimulus shows larger depressogenic ‘gain' in the context of a sensitizing genotype). This approach could be extended to use an instrumental variables analysis (for example, a Mendelian randomization study) to determine whether inflammatory signals function as mediators of, moderators of, or functionally independent additional additive influences with respect to, social precipitants of MDD.
Thus far, we have focused on the possibility that risk alleles promote depressive symptoms primarily as a result of increasing activity in inflammatory and/or immune-relevant downstream physiological pathways (that is, gene → immune effects → depression). However, many of the associations cited in this review could be equally well accounted for by the hypothesis that alleles directly influence CNS functioning to increase MDD risk, and that MDD subsequently affects immune function (that is, gene → depression → immune effects). This possibility is especially likely for genes such as NPY
, which we have described in immune terms, but that also has well-documented effects on CNS functioning relevant to depression. In addition to downstream immune effects, such genes may also have enhanced host defense in ancestral environments by promoting behavioral patterns likely to reduce the risk of becoming infected, spreading infection to kin or of dying once an infection had commenced.395, 396
Just such effects have been proposed for the short allele of the serotonin transporter, which has been associated with collectivistic social behavior relevant to host defense.104
Immune changes associated with MDD are not only specific but also occur in other severe mental disorders, including bipolar disorder and schizophrenia. Although the high prevalence of depression in these conditions is consistent with a PATHOS-D perspective, it is hard to imagine that other behavioral states associated with these diseases, including mania and psychosis, are adaptive for pathogen host defense. Indeed, the impaired decision-making characteristic of both states and the social isolation/reduced access to resources that is common in psychosis would be expected to increase vulnerability to pathogen exposure. Given overlapping genetic risk factors for these conditions and MDD, it is possible that they are best understood as purely maladaptive states supported at relatively low levels in the human population, at least in part, because their genetic antecedents enhanced host defense in carriers of immune-relevant risk alleles who responded to infectious challenges with enhanced immune activation and sickness behavior/depression without developing the full disease phenotype. Another possibility is that very severe disorders such as bipolar disorder and schizophrenia have been maintained in the human genome because immune benefits accrued to afflicted individuals that counteracted the fitness-reducing behavioral profiles (including increased risk of infection) associated with these diseases. This scenario would suggest that immune changes seen in schizophrenia and bipolar disorder should be more robust than those seen in depression, because they would have to be large enough to offset behavioral costs not present in depression. Although not entirely consistent,397
some data support this possibility.398, 399, 400