Assessing the associations between plasma C reactive protein, Ab 42/40 ratio, global amyloid and entorhinal tau PET

Background Lower plasma ratio of amyloid beta (Ab42/Ab40) has been associated with brain amyloid, however, the mechanistic link is unclear. Systemic inflammation may contribute to plasma Ab levels and promote or reduce the accumulation of Alzheimer’s disease (AD) neuropathology. C reactive protein (CRP) is an inflammatory marker implicated in amyloidosis. Using a cross sectional analysis, we evaluated the relationship between blood levels of CRP and Ab ratio, global amyloid positron emission tomography (PET), and tau PET in the entorhinal cortex where tau accumulates early. We hypothesized that Ab positivity would modify the relationship between CRP and AD pathology. Method In 969 cognitively normal participants (age: 50‐87, 781 Ab‐/188 Ab+) (Table 1) from the Health and Aging Brain Study‐ Health Disparities (HABS‐HD), we acquired 18F‐PI‐2620 tau‐PET, florbetaben amyloid PET scans, and plasma measures of Ab 42/40 and CRP. Subjects were categorized by Ab + or Ab ‐ by a global amyloid PET standardized uptake value ratio (SUVR) cutoff of 1.08. Within Ab groups, we performed multiple regressions to evaluate associations between 1) global amyloid and entorhinal tau‐PET SUVRs and 2) plasma markers of Ab ratio and CRP. We covaried for age, gender, education, APOE4, and scanner and used false discovery rate (FDR) for multiple comparison corrections. Result In Ab+ participants, higher amyloid (b = ‐0.31, p = 0.0005) and tau PET (b = ‐0.36, p = 0.004) were significantly associated with low Ab ratio, but not in Ab‐ for amyloid or tau, with significant interactions by Ab positivity for amyloid (b = ‐0.45, p = 2.65×10−6) and tau (b = ‐0.77, p = 0.0002) (Figure 1). In Ab+ participants, higher CRP was associated with higher Ab ratio (b = 0.21, p = 0.04) and a trend toward lower amyloid (b = ‐0.15, p = 0.084) and tau PET (b = ‐0.22, p = 0.084), but not in Ab‐ participants, with a significant interaction by Ab positivity for Ab ratio (b = 0.46, p = 0.011) (Figure 2) and amyloid (b = ‐0.11, p = 0.0006) and tau PET (b = ‐0.17, p = 0.016). Conclusion In cognitively intact Ab+ but not Ab‐ adults, higher plasma Ab ratio was associated with lower global amyloid and entorhinal tau PET. In Ab+ participants, higher CRP was associated with blood and brain amyloid and tau levels consistent with less AD pathology.