Herpesvirus Infections in the Human Brain: A Neural Cell Model of the Complement System Derived from Induced Pluripotent Stem Cells

Background: Herpesviruses alter cognitive functions in humans following acute infections; progressive cognitive decline and dementia have also been suggested. It is important to understand the pathogenic mechanisms of such infections. The complement system – comprising functionally related proteins integral for systemic innate and adaptive immunity – is an important component of host responses. The complement system has specialized functions in the brain. Still, the dynamics of the brain complement system are still poorly understood. Many complement proteins have limited access to the brain from plasma, necessitating synthesis and specific regulation of expression in the brain; thus, complement protein synthesis, activation, regulation, and signaling should be investigated in human brain-relevant cellular models. Cells derived from human-induced pluripotent stem cells (hiPSCs) could enable tractable models. Methods: Human-induced pluripotent stem cells were differentiated into neuronal (hi-N) and microglial (hi-M) cells that were cultured with primary culture human astrocyte-like cells (ha-D). Gene expression analyses and complement protein levels were analyzed in mono- and co-cultures. Results: Transcript levels of complement proteins differ by cell type and co-culture conditions, with evidence for cellular crosstalk in co-cultures. Hi-N and hi-M cells have distinct patterns of expression of complement receptors, soluble factors, and regulatory proteins. hi-N cells produce complement factor 4 (C4) and factor B (FB), whereas hi-M cells produce complement factor 2 (C2) and complement factor 3 (C3). Thus, neither hi-N nor hi-M cells can form either of the C3-convertases – C4bC2a and C3bBb. However, when hi-N and hi-M cells are combined in co-cultures, both types of functional C3 convertase are produced, indicated by elevated levels of the cleaved C3 protein, C3a. Conclusions: hiPSC-derived co-culture models can be used to study viral infection in the brain, particularly complement receptor and function in relation to cellular “crosstalk.” The models could be refined to further investigate pathogenic mechanisms.