The race to produce vaccines against SARS-CoV-2 began when the first sequence was published, and this forms the basis for vaccines currently deployed globally. Independent lineages of SARS-CoV-2 have recently been reported: UK–B.1.1.7, South Africa–B.1.351 and Brazil–P.1. These variants have multiple changes in the immunodominant spike protein which facilitates viral cell entry via the Angiotensin converting enzyme-2 (ACE2) receptor. Mutations in the receptor recognition site on the spike are of great concern for their potential for immune escape. Here we describe a structure-function analysis of B.1.351 using a large cohort of convalescent and vaccinee serum samples. The receptor binding domain mutations provide tighter ACE2 binding and widespread escape from monoclonal antibody neutralization largely driven by E484K although K417N and N501Y act together against some important antibody classes. In a number of cases it would appear that convalescent and some vaccine serum offers limited protection against this variant.

he SARS-CoV-2 virus is thought, based on sequence identity, to have crossed from bats to humans in 2019 1 . Similar to SARS-CoV-1 (2002-2003 and MERS-CoV (2012), SARS-CoV-2 presents as a respiratory disease but can progress into internal organs and cause organ failure 2,3 . A recent report from France estimates a fatality rate of 0.7% and a hospitalization rate of 3.6% 4 . Both these rates are much higher in elderly populations 4,5 . Around 33% of those admitted to UK hospitals with COVID-19 have died 6 . Because SARS-CoV-2 also spreads rapidly in the naive human population 7 , the current COVID-19 pandemic has presented an unprecedented challenge to modern human society. Although there is currently no 'cure' or vaccine for the disease, passive immune therapy by transfusing critically ill COVID-19 patients with serum from COVID-19 convalescent individuals has been shown to improve clinical outcomes 8,9 . This would suggest that neutralization of the virus, even at a relatively late stage in the disease, may be a useful COVID-19 therapy.The single-positive-strand RNA genome of SARS-CoV-2, like SARS-CoV, encodes four major structural proteins: spike, envelope, membrane and nucleocapsid. The spike protein comprises an N-terminal (S1) subunit, which contains the roughly 200-residue receptor binding domain (RBD) 10,11 , and a C-terminal subunit (S2), which contains the fusion protein 12 (Fig. 1a). The RBD of SARS-CoV-2 binds more tightly to the extracellular domain of angiotensin-converting enzyme 2 (ACE2) (Fig. 1a) than the homologous SARS-CoV-1 RBD 13 . The higher affinity results from sequence changes in RBD (Fig. 1b) and this has been proposed to underlie the higher transmissibility of SARS-CoV-2 14 . Antibodies raised to the spike protein of SARS-CoV-1 can neutralize the virus both in vitro and in vivo, by binding to the RBD and blocking binding to ACE2 15 . Unfortunately, most of these antibodies do not cross-react with the SARS-CoV-2 RBD 13 . The CR3022 antibody derived from a convalescent SARS-CoV-1 patient is cross-reactive to both SARS-CoV-1 and SARS-CoV-2 RBD (reported apparent K D of 6 nM, ref. 16 ). Two studies have reported crystal structures of CR3022 bound to SARS-CoV-2 RBD and show that the target epitope is distant from the ACE2 binding region 17,18 , which is consistent with the observation that CR3022 does not block RBD binding to ACE2. Another study on CR3022 has reported highly effective SARS-CoV-2 neutralizing activity that appears to arise from destabilization of the spike trimer, a novel mechanism for neutralizing SARS-CoV-2 18 . Destabilization of viral proteins by antibodies has been observed for influenza 19 and human immunodeficiency virus 20 .Mammalian, including human, antibodies generally have two chains (heavy and light), but camelids, in addition to two-chain antibodies, also possess a single-heavy-chain antibody variant 21 .

Highlights d Original strain convalescent and vaccine sera show reduced B.1.1.7 neutralization d N501Y enhances RBD: ACE2 binding affinity d N501Y compromises neutralization by many antibodies with public V-region IGHV3-53 d No widespread escape by B.1.1.7 was observed

SummaryMicroglia are increasingly implicated in brain pathology, particularly neurodegenerative disease, with many genes implicated in Alzheimer's, Parkinson's, and motor neuron disease expressed in microglia. There is, therefore, a need for authentic, efficient in vitro models to study human microglial pathological mechanisms. Microglia originate from the yolk sac as MYB-independent macrophages, migrating into the developing brain to complete differentiation. Here, we recapitulate microglial ontogeny by highly efficient differentiation of embryonic MYB-independent iPSC-derived macrophages then co-culture them with iPSC-derived cortical neurons. Co-cultures retain neuronal maturity and functionality for many weeks. Co-culture microglia express key microglia-specific markers and neurodegenerative disease-relevant genes, develop highly dynamic ramifications, and are phagocytic. Upon activation they become more ameboid, releasing multiple microglia-relevant cytokines. Importantly, co-culture microglia downregulate pathogen-response pathways, upregulate homeostatic function pathways, and promote a more anti-inflammatory and pro-remodeling cytokine response than corresponding monocultures, demonstrating that co-cultures are preferable for modeling authentic microglial physiology.

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