Mounting and compelling evidence suggest that systemic inflammatory process may play a central role in the development of Alzheimer’s disease (AD) (McGeer and McGeer, 1998; Kuo et al., 2005; Holmes, 2013; Heppner et al., 2015; Kinney et al., 2018). Inflammatory proteins [i.e., Interleukin-6 (IL-6) and C-reactive protein (CRP)] have been detected in brain plaques and neurofibrillary tangles in patients with AD (Iwamoto et al., 1994; Huell et al., 1995), suggesting the involvement of inflammatory processes in the etiology of AD. As an acute-phase reactant, CRP is a relatively stable but non-specific marker of systemic inflammation which is primarily regulated by IL-6 (Pepys and Hirschfield, 2003). Clinically, CRP concentration was associated with age-related diseases, and was used as a marker of cardiovascular risk (Verma et al., 2005). Notably, peripheral CRP levels was moderately correlated with other inflammatory cytokines such as IL-6, IL-1, tumor necrosis factor (TNF) (r = 0.232–0.537, all p < 0.05), and was associated with cerebrospinal fluid CRP (r = 0.855, p < 0.001), suggesting that peripheral CRP is a useful marker for both peripheral and central immune activity (Felger et al., 2020).

The progression of AD is mainly characterized by cognitive decline and brain atrophy (Kinney et al., 2018). Despite mounting of evidence suggest that increased CRP concentration may have a deleterious impact on cognitive function and future risk of AD on non-demented populations (Teunissen et al., 2003; Engelhart et al., 2004; Komulainen et al., 2007; Kravitz et al., 2009; Wersching et al., 2010), conflicting findings have been reported in patients diagnosed with AD dementia and patients with mild cognitive impairment (MCI) (Locascio et al., 2008; O’Bryant et al., 2010; Silverman et al., 2012; Fernandes et al., 2020; Lewis and Knight, 2021). For instance, Fernandes et al. (2020) found that lower CRP level predicted a faster conversion to future AD dementia in patients with mild cognitive impairment (MCI), suggesting that the impact of CRP on AD pathology may vary with the disease progression. Supporting this notion, in a recent 10-year longitudinal study, increased CRP levels has been linked to worse future cognitive function only among cognitively healthy older participants; while in patients with dementia, elevated CRP was associated with slower future cognitive decline (Lewis and Knight, 2021). This evidence raised the question of whether CRP concentration associates with different brain alterations patterns in the context of different disease progression. In fact, several neuroimaging studies attempted to explore these relationships. CRP concentration has been linked to greater white matter hyperintensities severity (a loss of brain vascular integrity due to axonal loss and demyelination) (Young et al., 2008; Satizabal et al., 2012; Walker et al., 2018), decreased fractional anisotropy (Wersching et al., 2010; Wooten et al., 2021), greater regional cerebral blood flow decline (Warren et al., 2018). Moreover, increased CRP concentration has been linked to smaller gray matter volumes (GMVs) in the medial temporal lobe and cingulate cortex (Bettcher et al., 2012; Taki et al., 2013; Marsland et al., 2015), which may explain the association between CRP level and decreased cognitive function in cognitively healthy older adults (Teunissen et al., 2003; Engelhart et al., 2004; Komulainen et al., 2007; Kravitz et al., 2009; Wersching et al., 2010). However, it should be noted that these findings were lacking replication in patients already with cognitive impairment. Moreover, these studies employed whole-brain exploratory analysis, which is limited in lacking specificity of AD and immune-related neuroanatomical changes. Furthermore, the relationship between CRP and AD risk may be largely confounded by cardiovascular disease (CVD), which is highly associated with peripheral CRP level (Koyama et al., 2013). In a previous study, participants with increased CRP concentration at baseline increased over 50% risk of developing dementia; while this association no longer remained significant after adjusting for possible confounding of CVD risk factors (Hsu et al., 2017). Therefore, a strict exclusion based on detailed medical history are needed to control possible confounding variables, such as CVD and other diseases.

The present study had two main goals: (1) to investigate whether CRP is associated with brain structural alterations in the context of AD [patients with subjective cognitive decline (SCD), patients with MCI and patients with AD], and (2), whether CRP is associated with cognitive changes among patients with different stages of cognitive impairments. Findings from present study may contribute to mechanistic understanding of association between systemic inflammation, cognitive changes, and brain structural alterations in AD.

This study aimed to investigate hs-CRP concentration, brain structural alterations, and cognitive function in the context of AD. We observed a significantly positive association between CRP concentration and GMVs in the left fusiform and left parahippocampal gyrus. We also investigated whether hs-CRP was associated with cognitive function. We observed a significant interaction between diagnosis and CRP concentration in predicting the annual cognitive changes; post hoc analysis revealed that among these patients diagnosed with AD, lower baseline hs-CRP was correlated with greater future cognitive decline. Collectively, our findings suggest that increased hs-CRP may have exert protective effect on brain structure alterations and future cognitive changes among patients already with cognitive impairment.

Elevated hs-CRP concentration was associated with increased GMVs in the left fusiform and left parahippocampal gyrus. Bettcher et al. (2012) reported that compared with undetectable group, participants with detectable CRP level (>3 mg/dL) showed smaller GMVs in the medial temporal lobe. It is noteworthy that these participants were non-demented and only the medial temporal lobe was selected. In present study, we selected 24 brain regions which has been previously linked to progression of AD and systemic inflammation. Additionally, the post-hoc tests revealed that the positive association between CRP level and GMVs was mainly driven by patients with cognitive impairment, that is, patients with MCI or patients diagnosed with AD. The fusiform, known as “the face area,” is critical for face perception (Kanwisher et al., 1997). Impaired facial recognition, e.g., fail to identify family members, is a common symptom in patients with AD (Hargrave et al., 2002). Amyloid deposition in the hippocampus has been linked to decreased GMVs in fusiform gyrus, suggesting an upstream and downstream relationship between these two areas (Chang et al., 2016). As part of the medial temporal lobe, the parahippocampal gyrus is mainly involved in memory processing (Köhler et al., 1998), which is a useful biomarker in neuropathology of AD (Echávarri et al., 2011; van Velzen et al., 2020). Notably, brain atrophy in the parahippocampal gyrus has been proposed to be a better predictor than hippocampus in the very early stages of AD (Echávarri et al., 2011). Our results raise the possibility that increased hs-CRP concentration may exert neuroprotective effect on brain structure alterations among patients who already with cognitive impairment, that is, increased CRP may represent an active immune function to against the progression of AD (Silverman et al., 2012). This hypothesis was proposed by Silverman et al. (2012), which they found a replicable association between increased CRP in proband and decreased dementia risk in their relatives (Silverman et al., 2012), suggesting the protective genotypes may moderate impact of systemic inflammation on AD. Supporting this notion, immune-related gene expression was found to be downregulated in subjects with cognitive impairment, while it was upregulated in the brains of cognitively healthy individuals (Katsel et al., 2009). This suggests that a robust immune system plays a central role in preserving cognitive function, whereas an inability to respond to the immune system could contribute to the disease progression (Katsel et al., 2009).

We also found a significant interaction between CRP concentration and diagnosis in predicting future cognitive decline. Specifically, increased CRP concentration at baseline was associated with slower cognitive decline in patients with AD. Our result was consistent with previous studies based on patients who already diagnosed with AD (Locascio et al., 2008; O’Bryant et al., 2010; Lima et al., 2014; Gong et al., 2016; Lewis and Knight, 2021; Saxton et al., 2023). For instance, Locascio et al. (2008) reported that a lower CRP level was associated with both greater cognitive and functional decline in patients with AD. In a recent large longitudinal study, researchers found that increased CRP was associated with worse baseline cognitive function in non-demented participants; while among participants with dementia, elevated CRP was associated with slower cognitive decline (Lewis and Knight, 2021), suggesting the impact of CRP on cognition may vary with the disease progression (O’Bryant et al., 2010; Gong et al., 2016; Fernandes et al., 2020). Despite more evidence is warranted to elucidate how CRP affects cognitive function in the neuropathology of AD, our result suggests that increased CRP level may indicate an appropriate immune response function among patients who already with cognitive impairment, contributing to slower cognitive decline in the AD progression. Our findings could also explain why epidemiological studies suggest the impact of increased inflammation on AD could be harmful; while the clinical treatments targeting on anti-inflammatory process have failed (Aisen, 2002; Wang et al., 2015; Ozben and Ozben, 2019), presumably due to a lack of considering for the different stage of progression. From this perspective, our results underline the importance of anti-inflammation treatments target on AD that should consider the different stages of this disease.

The mechanisms underlying the protective effect of CRP concentration on brain alterations and cognitive changes in patients with MCI and AD remain unclear. One potential explanation is that the presence of the Apolipoprotein ε4 (APOE ε4) allele, a well-studied genetic risk factor for AD, may modify the impact of CRP on AD-related neurodegeneration. Both animal and human studies have suggested APOE ε4 plays a role in the inflammatory process (Jofre-Monseny et al., 2008; Rebeck, 2017; Tzioras et al., 2019). It has been observed that individuals with APOE ε4 carriers exhibit lower CRP concentrations compared to non-APOE ε4 carriers, indicating potential variations in immune function among individuals with different APOE ε4 alleles (Haan et al., 2008; Hubacek et al., 2010; Tao et al., 2018; Tao et al., 2021; Wooten et al., 2021; Wang et al., 2022). Studies have reported the neuroprotective effect of increased CRP levels in both individuals without the APOE ε4 allele (Silverman et al., 2008; Lima et al., 2014; Tao et al., 2021) and APOE ε4 carriers (Haan et al., 2008). These findings may be attributed to differences in disease progression and demographic characteristics. Notably, elevated CRP levels have been associated with lower cortical Aβ specifically in APOE ε4 carriers, suggesting that increased CRP may reflect an adaptive function within this APOE ε4 genotype, contributing to improved plaque clearance (Hooper et al., 2018). However, it has also been reported that increased CRP levels have a detrimental effect on cognition in APOE ε4-positive individuals (Tao et al., 2021; Wooten et al., 2021). Further studies are needed to clarify whether APOE ε4 plays a mediating role in the association between systemic inflammation and the neuropathology of AD. Another potential explanation involves the activation of microglia. Specifically, an increased concentration of CRP may activate microglia, leading to a neurotrophic effect on the brain and cognition in patients with cognitive impairments. Peripheral inflammation can induce an inflammatory process in the brain through the activation of microglia (Riazi et al., 2008; Norden and Godbout, 2013). While sustained activation of microglia can have a neurotoxic effect and contribute to cognitive deficits, it is important to note that activated microglia can also be beneficial by producing neurotrophic compounds (Sawada et al., 2008). Notably, the transition from a neuroprotective effect to a neurotoxic effect of activated microglia depends on various factors, including disease progression and brain regions (Sawada et al., 2008). Although not directly linked to AD, the characteristics of primed microglia in individuals with Parkinson’s disease (PD) and dementia with Lewy bodies (DLB) suggest variations in the secretion of cytokines and neurotrophins, potentially resulting in neuroprotective effects in PD and neurotoxic effects in DLB (Imamura et al., 2005). It is worth mentioning that CRP can inhibit the production of nitric oxide, a neurotoxic substance produced by microglia (Chao et al., 1992). Further investigation is warranted to unravel the precise mechanisms underlying the interplay between increased CRP levels, microglial activation, and cognitive function in patients with AD and MCI.

Several limitations must be mentioned. First, despite we excluded possible confounding variable based on the detailed medical history, we cannot rule out the possibility that medication status contribute to the impact of CRP on different stages of AD. For example, the use of anti-inflammatory drugs, e.g., NSAID, could have impact on the CRP concentration. Second, our study was based on a limited sample size. We only included 313 clinical patients and 91 patients with longitudinal cognitive scores. Future studies could investigate systemic inflammation, brain alterations and cognitive changes with a larger sample size. Moreover, since we did not follow all patients, a selective bias should be considered in the longitudinal analysis. Last, we failed to include other inflammatory markers, such as TNF and IL-6 in this study. Although CRP is a sensitive marker that had been used in many previous studies, proinflammatory markers may accurately reflect the inflammatory status. Future research could examine the relationships between a broader range of inflammatory markers, brain alterations and cognitive changes in individuals with AD.

The original contributions presented in this study are included in this article/Supplementary material, further inquiries can be directed to the corresponding author.

Approval for present study was acquired by the Clinical Research Ethics Committee of the Tohoku University Graduate School of Medicine (approval number: 2018-1-618). Written informed consent to participate in this study was provided by the participants’ legal guardian/next of kin. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

YZ and YKT co-designed the study. YKT was responsible for scanning the subjects and supporting the image technology. YKT, TNK, TNG, MM, and YT collected the data. YZ, YL, and BT analyzed the data. YZ wrote the first draft of the manuscript. YYT supervised this study. All authors have contributed to and approved the final manuscript.