Using Western blots, IFA and IHC, we have confirmed an almost silent proliferation of microglia in the brain tissues of human genetic prion diseases, including FFI and G114V gCJD, but an active proliferation of microglia in the brains of sCJD. Morphologically, the brain microglia cells change their cell sizes from small cell bodies with a few fibrils in normal control and genetic prion diseases to larger amoeboid soma with numerous fibers in sCJD cases. Moreover, the levels of several cytokines, including IL-1β, IL-6 and TNF-α, in the brains of sCJD show significantly increased, possibly reflecting an activated situation of microglia, while those in the brains of FFI cases and G114V gCJD case maintain comparable as normal control. Our data here may highlight distinct responses of CNS tissues when exposing to various subtypes of prions.
Human genetic prion diseases consist of three subtypes, gCJD, FFI and GSS. Although all of them are dominant genetic diseases, they may vary largely in the clinical manifestations, neuropathological features and PrPSc characteristics, due to the different mutations in PRNP and the polymorphism of the amino acid at codon 129. Previous literatures have repeatedly identified upregulation in microglial response in GSS, where microglial activation often accompanies prion protein deposition and neuronal loss [3, 7, 8]. Unlike GSS characterized pathologically by large amounts of amyloid deposits that mostly localize in the cerebral and cerebellar cortices and the basal ganglia, FFI usually lacks of observable PrPSc deposit, instead of extensive gliosis in thalamus, which might associate with the inactivation of microglial cells in FFI brains. Interestingly, although obviously PrPSc deposits are detected in the brains of G114V gCJD in our study, the brain microglial cells seems to be silent. The exact reason remains unknown. It cannot be excluded that the presence of different subtypes of prion strains among the various genetic prion diseases might affect the activation of microglia, irrespective of the presence of amyloid deposits Actually, in a set of animal models of CJD, different microglial activations have been already observed .
As resident immune cells in CNS, microglia may serve as an agent of immune surveillance and host defense that sensitively responses to the microenvironmental changes induced by neuronal injuries and infections . Microglia activations in CNS tissues have been repeatedly identified in naturally-occurred scrapie in sheep and many scrapie-infected rodent models [11–15], as well as in bovine spongiform encephalopathy [16, 17]. Similarly, activated microglial cells are also observed in the brain tissues, especially in the plaques of human infectious prion diseases, such vCJD and Kuru [18, 19]. Meanwhile, microglia/macrophage induced inflammatory cytokines are increasingly released in the CNS tissues of various infectious human and animal TSEs [15, 20]. Those data adequately illustrate that the host local immune defense system is activated during prion infections, by prion itself or/and amyloid plaques. Activations of microglia in CNS are also frequently observed in sCJD patients [6, 19, 21–23], though one study has proposed that less microglial cells are detected in the plaques of sCJD than those in vCJD and GSS, even less than Kuru . The exact etiology of sCJD remains still unsettled, although it is usually considered due to the spontaneously conversion from PrPC to PrPSc with unknown reason. However, the contact of exogenous infectious agent during long life-span of human being for sCJD cannot be absolutely excluded so far. Contrast to the acquired and sporadic forms of human prion diseases, the inherited human prion diseases, such as FFI and G114V gCJD in this study, are caused by the special mutated PrPs that usually show little infectivity in bioassays. One may speculate that besides of the formation amyloid plaque (for GSS and Alzheimer’s disease, AD), the subtypes of infectious human prions may also contribute to activation of microglia in CNS. In line with the description previously , our data here also show more activated microglia in sCJD with type 1 PrPSc.
Besides of deposits of PrPSc, neuron loss, astrogliosis and spongiform degeneration are also hallmarks for TSEs. However, those markers seem not to be associated with the level and status of microglial cells in the brains of FFI and G114V gCJD cases. More PrPSc deposits and more extensive spongiform have been observed in the cortex regions of the G114V gCJD case , while more severe gliosis have been observed in the regions of thalamus in those three FFI cases . However, the levels of Iba1 positive signals among the ten tested brain regions from either FFI or G114V gCJD are quite comparable. No detectable plaque and extremely low amounts of PrPSc (PK-resistant PrP in Western blots) in FFI cases may relate to silent brain microglia. The brain tissues of G114V gCJD contain large amounts of PrPSc that is almost comparable with that of sCJD, but appear very limitedly increased microglia. It indicates again that the activation of microglia during prion pathogenesis may vary depending on the prion strains. Moreover, in addition to its effect of agent clearance, activation microglia also possibly contributes to enhance the neuronal destruction . Apoptotic neurons in CJD are probably related to the presence of inflammatory cells and cytokines which are present during the whole CJD disease process . Lack of or very limited activated microglia in the CNS tissues of FFI and G114 gCJD suggest that recruitment of inflammatory cells is not the major reason for neuronal destruction for these two inherited prion diseases, which highlight again the diversity of the pathogenesis of human prion diseases [1, 27].