Periodic Reporting for period 4 - Bat Flu (Deciphering the unconventional receptor binding and modulation activity of bat influenza A viruses)
Reporting period: 2025-05-01 to 2025-10-31
Influenza A viruses (IAVs) are zoonotic pathogens that can cause severe diseases and pandemics. Initially, it was believed that all hemagglutinin (HA) proteins of IAVs bind sugar residues (sialic acids) for cell entry. However, the discovery of the two new IAVs H17N10 and H18N11 in bats in Central and South America challenged this notion. These viruses use major histocompatibility complex class II (MHCII) proteins instead of sialic acids to enter cells. Interestingly, these bat-derived viruses can also use human MHCII proteins for host cell entry, possibly enabling zoonotic spill-over infections. However, it remained unclear how these viruses recognize MHCII to initiate the entry process and which cell types they infect in their natural bat host reservoir. To shed light on this new mechanism of IAV entry and cell tropism, the BatFLu project aimed to gain mechanistic insights into how these bat IAVs engage MHCII as an entry factor, identify the cell types infected in bats, and determine whether human cells are also susceptible to infection with bat IAVs. This information, especially the latter, will help unveil their potential to spread to the human population.
We used functional cell-based assays combined with a mutational approach to show that specific regions of MHCII and H18 are important for cell entry and probably correspond to binding sites that interact with each other. We also showed that a region in HA, distinct from the classical sugar binding region, is required for MHCII interaction. However, the affinity of this interaction is too low to be detected by standard biochemical binding assays. Therefore, we speculated that the viral particle increases the affinity by binding to multiple MHCII proteins. Using a new live imaging approach, we demonstrated that bat IAV particles bind to pre-formed clusters of MHCII molecules and subsequently induce an increase in cluster size by attracting additional molecules. Thus, these bat IAV particles compensate for the weak affinity of the individual HA/MHCII interaction by clustering MHCII molecules. MHCII is expressed on professional antigen-presenting cells (APCs). Single-cell sequencing of the intestines and associated lymph nodes of the natural bat host species infected with H18N11 revealed that enterocytes and leukocytes, particularly macrophages, become infected. Likewise, human APCs, particularly macrophages, are highly susceptible to H18N11 infection. These findings highlight that, in principle, bat IAVs have the potential to cross the human species barrier.
We also observed in H18N11-infected bats a rapid expansion of B cells during the course of infection, suggesting that antibodies might be relevant for controlling H18N11. Indeed, we showed that high neutralizing antibodies are generated in these animals after infection that were associated with no detectable viral shedding after secondary infection. These findings revisited the concept that viral infections of bats do mount significant levels of neutralizing antibodies.
During the course of the BatFLu project, we characterized the pandemic potential of bat H9N2, a recently discovered, bat-derived IAV circulating in Africa. Through a collaborative effort, we demonstrated that this virus has a greater ability than expected to cross the human species barrier due to efficient replication in human lung explants, transmission among ferrets, and escape of the human immune response. However, as with H17N10 and H18N11, there is currently no evidence of human infections with bat H9N2.
The major findings were disseminated to the public by several publications, press releases and presentations.
We also observed in H18N11-infected bats a rapid expansion of B cells during the course of infection, suggesting that antibodies might be relevant for controlling H18N11. Indeed, we showed that high neutralizing antibodies are generated in these animals after infection that were associated with no detectable viral shedding after secondary infection. These findings revisited the concept that viral infections of bats do mount significant levels of neutralizing antibodies.
During the course of the BatFLu project, we characterized the pandemic potential of bat H9N2, a recently discovered, bat-derived IAV circulating in Africa. Through a collaborative effort, we demonstrated that this virus has a greater ability than expected to cross the human species barrier due to efficient replication in human lung explants, transmission among ferrets, and escape of the human immune response. However, as with H17N10 and H18N11, there is currently no evidence of human infections with bat H9N2.
The major findings were disseminated to the public by several publications, press releases and presentations.
The findings of our virus-host studies provided significant advances in both IAV research and our understanding of bat immunology. Identifying intestinal leukocytes as the primary cellular target of H18N11 showed that IAVs can infect a broader range of cells than previously thought. Notably, H18N11 infects immune cells without triggering immunopathology, which could facilitate new insights into treating life-threatening courses of disease caused by emerging viral infections. In addition, characterizing the bat immune response after bat IAV infection revealed a robust antibody-mediated neutralizing immune response, challenging the common notion in the field of bat immunology.
The first step in the viral replication cycle is the attachment of the viral particle to the host cell surface. However, this initial event proceeds rapidly and is immediately followed by the uptake of the virus into the host cell. Therefore, the molecular dynamics occurring during viral attachment are nearly impossible to resolve. In the context of BatFLu, we developed a novel "inverse attachment assay" to study the movement of cellular receptors and viral particles, independent of internalization, using live, high-resolution microscopy. This technique is not restricted to bat IAV; it can be used to study the attachment of other viruses to their host cells and may lead to significant future discoveries in this field.
Because bats are the source of many human pathogens, it is essential to screen bat-borne viruses for their zoonotic and pandemic potential. BatFLu played an important role in the initial characterization of a bat-derived H9N2 strain circulating in African bats. The bat H9N2 virus shares characteristics with pandemic viruses and therefore requires further surveillance in pandemic preparedness programs.
The first step in the viral replication cycle is the attachment of the viral particle to the host cell surface. However, this initial event proceeds rapidly and is immediately followed by the uptake of the virus into the host cell. Therefore, the molecular dynamics occurring during viral attachment are nearly impossible to resolve. In the context of BatFLu, we developed a novel "inverse attachment assay" to study the movement of cellular receptors and viral particles, independent of internalization, using live, high-resolution microscopy. This technique is not restricted to bat IAV; it can be used to study the attachment of other viruses to their host cells and may lead to significant future discoveries in this field.
Because bats are the source of many human pathogens, it is essential to screen bat-borne viruses for their zoonotic and pandemic potential. BatFLu played an important role in the initial characterization of a bat-derived H9N2 strain circulating in African bats. The bat H9N2 virus shares characteristics with pandemic viruses and therefore requires further surveillance in pandemic preparedness programs.