SUMMARY Tissue-resident memory T (Trm) cells provide enhanced protection against infection at mucosal sites. Here we found that CD4+ T cells are important for the formation of functional lung-resident CD8+ T cells after influenza virus infection. In the absence of CD4+ T cells, CD8+ T cells displayed reduced expression of CD103 (Itgae), were mislocalized away from airway epithelia, and demonstrated an impaired ability to recruit CD8+ T cells to the lung air-ways upon heterosubtypic challenge. CD4+ T cell-derived interferon-γ was necessary for generating lung-resident CD103+ CD8+ Trm CD8 T cells. Furthermore, expression of the transcription factor T-bet was increased in “unhelped” lung Trm cells, and a reduction in T-bet rescued CD103 expression in the absence of CD4+ T cell help. Thus, CD4+ T cell-dependent signals are important to limit expression of T-bet and allow for the development of CD103+ CD8+ Trm cells in the lung airways following respiratory infection.

The presentation of viral epitopes to cytotoxic T lymphocytes (CTLs) by swine leukocyte antigen class I (SLA I) is crucial for swine immunity. To illustrate the structural basis of swine CTL epitope presentation, the first SLA crystal structures, SLA-1*0401, complexed with peptides derived from either 2009 pandemic H1N1 (pH1N1) swine-origin influenza A virus (S-OIV NW9 ; NSDTVGWSW) or Ebola virus (Ebola AY9 ; ATAAATEAY) were determined in this study. The overall peptide-SLA-1*0401 structures resemble, as expected, the general conformations of other structure-solved peptide major histocompatibility complexes (pMHC). The major distinction of SLA-1*0401 is that Arg 156 has a "one-ballot veto" function in peptide binding, due to its flexible side chain. S-OIV NW9 and Ebola AY9 bind SLA-1*0401 with similar conformations but employ different water molecules to stabilize their binding. The side chain of P7 residues in both peptides is exposed, indicating that the epitopes are "featured" peptides presented by this SLA. Further analyses showed that SLA-1*0401 and human leukocyte antigen (HLA) class I HLA-A*0101 can present the same peptides, but in different conformations, demonstrating cross-species epitope presentation. CTL epitope peptides derived from 2009 pandemic S-OIV were screened and evaluated by the in vitro refolding method. Three peptides were identified as potential cross-species influenza virus (IV) CTL epitopes. The binding motif of SLA-1*0401 was proposed, and thermostabilities of key peptide-SLA-1*0401 complexes were analyzed by circular dichroism spectra. Our results not only provide the structural basis of peptide presentation by SLA I but also identify some IV CTL epitope peptides. These results will benefit both vaccine development and swine organ-based xenotransplantation.