CD8 T Cell Memory Recall Is Enhanced by Novel Direct Interactions with CD4 T Cells Enabled by MHC Class II Transferred from APCs

<div><p>Protection against many intracellular pathogens is provided by CD8 T cells, which are thought to need CD4 T cell help to develop into effective memory CD8 T cells. Because murine CD8 T cells do not transcribe MHC class II (MHC-II) genes, several models have proposed antigen presenting cells (APCs) as intermediaries required for CD4 T cells to deliver their help to CD8 T cells. Here, we demonstrate the presence of MHC-II molecules on activated murine CD8 T cells in vitro as well as in vivo. These MHC-II molecules are acquired via trogocytosis by CD8 T cells from their activating APCs, particularly CD11c positive dendritic cells (DCs). Transferred MHC-II molecules on activated murine CD8 T cells were functionally competent in stimulating specific indicator CD4 T cells. CD8 T cells that were “helped” in vitro and subsequently allowed to rest in vivo showed enhanced recall responses upon challenge compared to “helpless” CD8 T cells; in contrast, no differences were seen upon immediate challenge. These data indicate that direct CD8∶CD4 T cell interactions may significantly contribute to help for CD8 T cells. Furthermore, this mechanism may enable CD8 T cells to communicate with different subsets of interacting CD4 T cells that could modulate immune responses.</p> </div

In vitro interactions improve the in vivo recall response of CD8 T cells.

<p>P14 cells were specifically activated in vitro using gp33-41 peptide in the presence of either ova323-339 (OTII) or gp61-81 (SMARTA) peptide for 24 hrs. Activated CD8 T cells were then magnetically isolated and cultured with experienced CD4 T cells for another 24 hrs. Activated CD8 T cells were again magnetically isolated and adoptively transferred into WT mice. Animals were challenged with 5×10⁴ c.f.u. of Lmgp33 i.v. at <b>a</b>) 1 day (day +1) or <b>b</b>) 30 days (day +30) after transfer. Stimulation for ICS and flow cytometry analysis was performed on spleens taken 4 days after Lmgp33 challenge. Events were gated on live CD19−Thy1.1+CD8+. **p = 0.0079. Plots are representative from one of at least two independent experiments.</p

MHC-II is transferred onto CD8 T cells from APCs.

<p><b>a.</b> I-A^b and I-E^k staining on magnetically enriched Flt3L-DCs from CIIKO, B6 and B6×B10.A mice. Events were gated on CD11c+ singlets. <b>b.</b> I-A^b vs I-E^k staining on activated P14 cells after 24 hrs of in vitro culture without peptide, with an irrelevant peptide (ova257-64) or cognate peptide (gp33-41). Events gated on live CD11c-Thy1.1+CD8+ singlets. Plots are representative from one of two independent experiments.</p

MHC-II is transiently present on responding transgenic CD8 T after infection.

<p><b>a.</b> MHC-II staining vs CFSE dilution on P14 cells from spleen at different times (0, 12, 24, 36, 42, 50 and 62 hrs) after infection with 2×10⁶ p.f.u. LCMV Arm i.v.. Plots are representative of duplicates. First row: I-A^b-APC staining. Second row: FMO control. Events gated on live CD19−CD11c−Thy1.1+CD8+ singlets. <b>b.</b> MHC-II (I-Ab), CD54 and CD69 expression on CD8 T cells in spleen, PLN and MLN at different times (0, 12, 24, 36, 42, 50 and 62 hrs) after infection with 2×10⁶ p.f.u. LCMV Arm i.v.. Each point is represented by the mean and SEM.</p