The role of alveolar macrophages in COVID-19 pathology

Thesis

Tissue-resident alveolar macrophages (AMs) are among the first immune cells to encounter SARS-CoV-2 and initiate a protective immune response. However, in severe COVID-19, macrophage responses become hyperinflammatory and dysregulated. While SARS-CoV-2 entry into macrophages is well-documented, the route of entry, their capacity to support viral replication, and their direct contribution to disease pathology remain unclear.

In this thesis, I used iPSC-derived macrophages as a model of tissue-resident AMs to define the mechanisms and consequences of SARS-CoV-2 interaction with macrophages. I show that macrophages efficiently internalise SARS-CoV-2 via non-specific mechanisms independent of ACE2 or high-affinity spike-binding receptors. Using single-molecule fluorescence in situ hybridisation (smFISH) and qRT-PCR, I demonstrate that macrophages do not support productive viral replication and instead degrade viral RNA.

Notably, I demonstrate that macrophages become permissive to productive infection when an alternative entry route is provided, including transgenic ACE2 expression or specific class 1/4 anti-RBD antibodies. I propose that antibody-mediated infection occurs through a cooperative mechanism involving Fab-dependent stabilisation of spike and Fcγ receptor-mediated virion tethering. In addition, I identify a clinically developed class 1/4 anti-RBD antibody with infectionpromoting activity. Under these conditions, viral replication is enhanced by inhibition of interferon signalling.

Functionally, macrophages release inflammatory cytokines in response to internalisation of ancestral SARS-CoV-2 but not Omicron variants. They present viral antigens to CD4⁺, but not CD8⁺, T cells, consistent with exogenous antigen processing. Importantly, viral exposure does not impair macrophage viability or key cellular functions. Collectively, these findings suggest that direct infection of alveolar macrophages is unlikely to be a major driver of COVID-19 pathology. Instead, macrophage dysregulation may arise indirectly through inflammatory mediators and epithelial cell damage.

Authors: Napier, A

• DOI: 10.5287/ora-2z4rnpmme
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: SARS-CoV-2; macrophage; COVID-19
• Subjects: Virology; Infectious Disease; Immunology