Key Points
IL-15 trans-presentation (TP) and Ag presentation rely on APC–T cell interaction.
We showed IL-15R subunit assembly upon TP and joint motion with MHC to the IS.
IL-15R and TCR do not amplify each other’s signaling; TP is self-sufficient.
Visual Abstract
Abstract
IL-15 plays a pivotal role in the long-term survival of T cells and immunological memory. Its receptor consists of three subunits (IL-15Rα, IL-2/15Rβ, and γc). IL-15 functions mainly via trans-presentation (TP), during which an APC expressing IL-15 bound to IL-15Rα presents the ligand to the βγc receptor-heterodimer on a neighboring T/NK cell. To date, no direct biophysical evidence for the intercellular assembly of the IL-15R heterotrimer exists. Ag presentation (AP), the initial step of T cell activation, is also based on APC–T cell interaction. We were compelled to ask whether AP has any effect on IL-15 TP or whether they are independent processes. In our human Raji B cell–Jurkat T cell model system, we monitored inter-/intracellular protein interactions upon formation of IL-15 TP and AP receptor complexes by Förster resonance energy transfer measurements. We detected enrichment of IL-15Rα and IL-2/15Rβ at the synapse and positive Förster resonance energy transfer efficiency if Raji cells were pretreated with IL-15, giving direct biophysical evidence for IL-15 TP. IL-15Rα and MHC class II interacted and translocated jointly to the immunological synapse when either ligand was present, whereas IL-2/15Rβ and CD3 moved independently of each other. IL-15 TP initiated STAT5 phosphorylation in Jurkat cells, which was not further enhanced by AP. Conversely, IL-15 treatment slightly attenuated Ag-induced phosphorylation of the CD3ζ chain. Our studies prove that in our model system, IL-15 TP and AP can occur independently, and although AP enhances IL-15R assembly, it has no significant effect on IL-15 signaling during TP. Thus, IL-15 TP can be considered an autonomous, Ag-independent process.
Footnotes
This work was supported by the intramural research program of the National Cancer Institute, National Institutes of Health (to T.A.W.); Grants GINOP-2.3.2-15-2016-00026, NN129371, and ANN135107 from the National Research, Development and Innovation Office, Hungary (to G.V.); Grant EFOP-3.6.3-VEKOP-16-2017-00009 cofinanced by the European Union and the European Social Fund (to A.B.); and the Deutscher Akademischer Austauschdienst and the Tempus Közalapítvány under Grant 273478 (to G.V. and K.T.).
Á.K., J.V., G.M., and K.J. performed measurements; Á.K. and N.S. prepared reagents and cell lines; Á.K., G.M., K.T., K.J., Z.B., A.B., and G.V. analyzed data; Á.K., G.V., and T.A.W. drafted the manuscript; all authors edited the manuscript; and G.V. and T.A.W. conceptualized the work.
The online version of this article contains supplemental material.
Abbreviations used in this article
- AP
- Ag presentation
- DC
- dendritic cell
- EGFP
- enhanced GFP
- FLIM
- fluorescence-lifetime imaging microscopy
- FRET
- Förster resonance energy transfer
- IRF
- instrument response function
- IS
- immunological synapse
- Kv
- voltage-gated potassium
- MHC II
- MHC class II
- mTOR
- mammalian target of rapamycin
- SEE
- Staphylococcus enterotoxin E
- TP
- trans-presentation
- Treg
- regulatory T
- Received March 22, 2021.
- Accepted September 10, 2021.
- Copyright © 2021 by The American Association of Immunologists, Inc.
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