Universität zu Köln

Daniel, Katharina ORCID: 0000-0003-4586-3398 (2026). Characterization of pre-existing humoral immunity to H5N1 clade 2.3.4.4b influenza viruses in unexposed individuals. PhD thesis, Universität zu Köln.

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Abstract

Influenza A viruses remain a major challenge for global public health. Beyond the annual burden of seasonal influenza, influenza A viruses circulating in animal populations represent an unpredictable risk, as they can give rise to novel variants with pandemic potential. Among these zoonotic variants, highly pathogenic avian H5N1 clade 2.3.4.4b influenza viruses are of major concern due to their rapid global spread in birds and their frequent detection in mammalian species. In particular, the 2024 outbreak of these viruses in dairy cattle in the United States, accompanied by repeated transmission to humans, represents a notable shift in host range, thereby increasing opportunities for zoonotic transmission and viral adaptation. Pre-existing immunity plays a key role in susceptibility to infection with emerging zoonotic variants and is an important component of pandemic risk assessment. Therefore, this thesis assessed pre-existing antibodies against the H5N1 clade 2.3.4.4b A/Texas/37/2024 isolate associated with the cattle outbreak in H5N1-naive individuals. For this purpose, a highly sensitive hemagglutinin(HA)-pseudovirus neutralization assay was initially established, which showed strong positive correlation with authentic virus neutralization and enabled quantification of functional antibody reactivity against this and other influenza strains. By applying this assay and complementary binding and neutralization assays, A/Texas/37/2024-reactive antibodies were detected in all 66 H5N1-naive individuals analyzed at the serum and plasma level. However, neutralizing titers against A/Texas/37/2024 were consistently lower compared to seasonal H1 and H3 viruses. To further dissect the underlying B cell response, H5-reactive memory B cells were isolated from six H5N1-naive individuals. Functional characterization of the encoded antibodies revealed highly potent A/Texas/37/2024- neutralizing antibodies in five out of the six individuals. Subsequent analyses showed that these antibodies cross-react with H1, compete with antibodies directed against the conserved stem region of HA and protect mice from lethal H5N1 challenge. Together, these findings indicate that the observed serum-level H5-reactivity is largely mediated by group 1 cross-reactive HA stem antibodies. In conclusion, this work demonstrates the presence of pre-existing cross-reactive antibodies to the H5N1 A/Texas/37/2024 isolate in individuals without prior exposure to this subtype. Further studies are required to better define the functional relevance of such antibody responses for protection against infection with emerging zoonotic influenza variants.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Daniel, Katharina katharina.daniel01@gmail.com https://orcid.org/0000-0003-4586-3398 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-810324
Date:	2026
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Medicine > Virologie > Institut für Virologie
Subjects:	Life sciences
Uncontrolled Keywords:	Keywords Language Influenza Virus UNSPECIFIED H5N1 UNSPECIFIED Humoral immunity UNSPECIFIED Pre-existing antibodies UNSPECIFIED
Date of oral exam:	16 July 2026
Referee:	Name Academic Title Klein, Florian Prof. Dr. Nothnagel, Michael Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/81032