Bats are natural reservoir hosts, harboring more than 100 viruses, some of which are lethal to humans. The asymptomatic coexistence with viruses is thought to be connected to the unique immune system of bats. MHC class I (MHC I) presentation is closely related to cytotoxic lymphocyte immunity, which plays an important role in viral resistance. To investigate the characteristics of MHC I presentation in bats, the crystal structures of peptide–MHC I complexes of Pteropus alecto, Ptal-N*01:01/HEV-1 (DFANTFLP) and Ptal-N*01:01/HEV-2 (DYINTNLVP), and two related mutants, Ptal-N*01:01/HEV-1_PΩL (DFANTFLL) and Ptal-N*01:01_ΔMDL/HEV-1, were determined. Through structural analysis, we found that Ptal-N*01:01 had a multi-Ala–assembled pocket B and a flexible hydrophobic pocket F, which could accommodate variable anchor residues and allow Ptal-N*01:01 to bind numerous peptides. Three sequential amino acids, Met, Asp, and Leu, absent from the α1 domain of the H chain in other mammals, were present in this domain in the bat. Upon deleting these amino acids and determining the structure in p/Ptal-N*01:01_ΔMDL/HEV-1, we found they helped form an extra salt-bridge chain between the H chain and the N-terminal aspartic acid of the peptide. By introducing an MHC I random peptide library for de novo liquid chromatography–tandem mass spectrometry analysis, we found that this insertion module, present in all types of bats, can promote MHC I presentation of peptides with high affinity during the peptide exchange process. This study will help us better understand how bat MHC I presents high-affinity peptides from an extensive binding peptidome and provides a foundation to understand the cellular immunity of bats. Key Points The bat MHC I exhibits distinctive structural characteristics. Significant differences exist in the peptidome between in and out of the cell. A cluster of uniquely inserted residues may help promote high-affinity peptide binding.

Feline immunodeficiency virus (FIV) infection in domestic cats is the smallest usable natural model for lentiviral infection studies. FLA-E*01801 was applied to FIV AIDS vaccine research. We determined the crystal structure of FLA-E*01801 complexed with a peptide derived from FIV (gag positions 40 to 48; RMANVSTGR [RMA9]). The A pocket of the FLA-E*01801 complex plays a valuable restrictive role in peptide binding. Mutation experiments and circular-dichroism (CD) spectroscopy revealed that peptides with Asp at the first position (P1) could not bind to FLA-E*01801. The crystal structure and in vitro refolding of the mutant FLA-E*01801 complex demonstrated that Glu ⁶³ and Trp ¹⁶⁷ in the A pocket play important roles in restricting P1D. The B pocket of the FLA-E*01801 complex accommodates M/T/A/V/I/L/S residues, whereas the negatively charged F pocket prefers R/K residues. Based on the peptide binding motif, 125 FLA-E*01801-restricted FIV nonapeptides (San Diego isolate) were identified. Our results provide the structural basis for peptide presentation by the FLA-E*01801 molecule, especially A pocket restriction on peptide binding, and identify the potential cytotoxic T lymphocyte (CTL) epitope peptides of FIV presented by FLA-E*01801. These results will benefit both the reasonable design of FLA-E*01801-restricted CTL epitopes and the further development of the AIDS vaccine. IMPORTANCE Feline immunodeficiency virus (FIV) is a viral pathogen in cats, and this infection is the smallest usable natural model for lentivirus infection studies. To examine how FLA I presents FIV epitope peptides, we crystallized and solved the first classic feline major histocompatibility complex class I (MHC-I) molecular structure. Surprisingly, pocket A restricts peptide binding. Trp ¹⁶⁷ blocks the left side of pocket A, causing P1D to conflict with Glu ⁶³ . We also identified the FLA-E*01801 binding motif X (except D)-(M/T/A/V/I/L/S)-X-X-X-X-X-X-(R/K) based on structural and biochemical experiments. We identified 125 FLA-E*01801-restricted nonapeptides from FIV. These results are valuable for developing peptide-based FIV and human immunodeficiency virus (HIV) vaccines and for studying how MHC-I molecules present peptides.

Currently, the structural characteristics of the swine major histocompatibility complex (MHC) class I molecule, also named swine leukocyte antigen class I (SLA-I) molecule need to be further clarified. A complex of SLA-I constituted by an SLA-2*HB01 molecule with swine β2-microglobulin and a cytotoxic T lymphocyte (CTL) epitope FMDV-AS64 (ALLRSATYY) derived from VP1 protein (residues 64–72) of Asia 1 serotype of foot-and-mouth disease virus (FMDV) was expressed, refolded, purified and crystallized. By preliminary X-ray diffraction analysis, it was shown that the diffraction resolution of the crystal was 2.4 Å and the space group belonged to P212121 with unit cell parameters a = 48.37, b = 97.75, c = 166.163 Å. This research will be in favor of illuminating the structural characteristics of an SLA-2 molecule associated with a CTL epitope derived from Asia1 serotype of FMDV.

As the most prevalent companion animal, cats are threatened by numerous infectious diseases and carry zoonotic pathogens such as Toxoplasma gondii and Bartonella henselae, which are the primary causes of human toxoplasmosis and cat-scratch disease. Vaccines play a crucial role in preventing and controlling the spread of diseases in both humans and animals. Currently, there are only three core vaccines available to prevent feline panleukopenia, feline herpesvirus, and feline calicivirus infections, with few vaccines available for other significant feline infectious and zoonotic diseases. Feline herpesvirus, a major component of the core vaccine, offers several advantages and a stable genetic manipulation platform, making it an ideal model for vaccine vector development to prevent and control feline infectious diseases. This paper reviews the technologies involved in the research and development of the feline herpesvirus vaccine vector, including homologous recombination, CRISPR/Cas9, and bacterial artificial chromosomes. It also examines the design and effectiveness of expressing antigens of other pathogens using the feline herpesvirus as a vaccine vector. Additionally, the paper analyzes existing technical bottlenecks and challenges, providing an outlook on its application prospects. The aim of this review is to provide a scientific basis for the research and development of feline herpesvirus as a vaccine vector and to offer new ideas for the prevention and control of significant feline infectious and zoonotic diseases.