Development of a modular chemical linking strategy for bispecific antibodies to suppress neuroinflammation in Alzheimer's disease via microglial polar

PhD project strategic theme: Biosciences for an integrated understanding of health

Microglia are the resident macrophages of the brain and due to the lack of antibodies in the brain, they are uniquely reactive amongst macrophages. Microglia can be polarised into two states, a pro-inflammatory, M1, and anti-inflammatory, M2 state, and can sit anywhere on the spectrum between the M1 and M2 states. In Alzheimer's disease (AD), microglial polarisation to a pro-inflammatory, M1 phenotype which is phagocytic, is mediated by pathogenic, misfolded proteins.

The complement system enables phagocytosis through multiple pathways (classical, lectin or alternative). Traditionally it was believed that the microglial complement system was utilised solely to clear pathogens and cellular debris. However, it has been found to have a crucial role in the proper maturation of neuronal circuits in the developing brain through synaptic pruning via opsonisation of neurons.

In AD, amyloid-beta plaques activate the complement pathway through polarisation of microglia to inflammatory M1 phenotypes which both activates the classical complement system and releases cytokines which activate reactive (A1) astrocytes, which mediate the alternative complement system. In combination, the aberrant activation of the microglial complement system leads to the opsonisation and concomitant phagocytosis of synapses and subsequent neurodegeneration in AD patients.

It has been shown that PD-1 deficiency induces pro-inflammatory M1 microglia after spinal cord injury (SCI) in PD-1 knockout mice. Whilst in wildtype mice, PD-1 expression is upregulated and anti-inflammatory M2 microglia were present, leading to better recovery from SCI. Based on this, we propose that upregulating PD-1 signalling in the AD brain, a reduction in neuroinflammation and neurodegeneration could be achieved through suppression of the microglial complement system.

Taking inspiration from cancer cells, which express PD-L1 to evade the immune response of T cells through activating the PD-1 signalling pathway, it is believed the microglial complement response could be suppressed in a similar way. By designing a bispecific antibody (bsAb) with an amyloid-beta plaque targeting domain and a PD-L1 domain, it is proposed that microglia which would migrate to amyloid-beta plaques and become polarised to the inflammatory M1 phenotype could be alternatively activated to the anti-inflammatory M2 state in the local environment surrounding the plaques. This would be achieved through the interaction of the PD-L1 domain of the bsAb with the PD-1 domain of the microglia, and thus suppress neuroinflammation.

The majority bsAbs currently in clinical trials are produced through recombinant expression of bsAbs using a flexible peptide to link the domains, however, this approach is not suitable for all bsAb constructs. Therefore, the intention is to design a robust chemical linking strategy that can be used not only for the model bsAb described but can also be applied to a number of bsAbs which are inaccessible using current expression based linking strategies. This will be achieved via a modular linking strategy, site-specifically targeting uniquely reactive residues which will be engineered into, or naturally occuring, in the separate protein domains. If successful, this approach could be extended to other disease models which benefit from the production bsAb constructs for targeted therapies, such as other neurodegenerative conditions or cancers.