MHC class I molecules usually present peptides that are derived from endogenous antigens. But they can also present peptides that are derived from exogenous antigens, through a process known as cross-presentation. Various mechanisms have been proposed to explain how this occurs. Now, Joost Neijssen and colleagues propose a novel mechanism — which they have named gap-junction-mediated immunological coupling (GMIC) — to add to the list.

Gap junctions are channels that form between adjacent cells. Each cell contributes a hemichannel of six connexin molecules to the functional gap junction. These channels allow the passive exchange of ions, nutrients and signalling components between cells. Haematopoietic cells express connexin-43, so the authors set out to investigate the role of gap junctions in peptide transfer between cells and whether this could contribute to cross-presentation.

First, the authors looked at whether peptides can be transferred between cells through gap junctions. The human squamous-cell carcinoma line A431, which does not form gap junctions, was stably transfected with connexin-43, resulting in the formation of gap junctions. Non-degradable, fluorescently labelled peptides were introduced into these cells, and peptide transfer was analysed by confocal microscopy. This showed that such transfer was possible and could be blocked using gap-junction inhibitors. The rate of peptide transfer was also determined and was shown to be inversely proportional to the size of the peptide: that is, the transfer rate was decreased for longer peptides.

Because the cytoplasm contains several cytosolic peptidases, the authors next looked at the impact of these peptidases on peptide transfer between cells. Using quenched peptides that become fluorescent after degradation, they found that cytosolic peptidases can limit, but not prevent, the spread of peptides from cell to cell.

So, does this peptide transfer have immunological relevance? To test this, the authors expressed an influenza-derived peptide in connexin-43-transfected A431 cells, which do not express HLA-A2, and they cultured these cells with HLA-A2-transfected A431 cells. When HLA-A2-restricted T cells specific for the influenza-derived peptide were added to the culture, a T-cell response was detected, showing that peptide transfer had occurred through gap junctions. Next, the authors carried out similar experiments using primary human monocytes. Connexin-43-transfected A431 cells were infected with influenza, and after 16 hours, viral propagation was inhibited. These infected cells were then cultured with primary human monocytes that had been stimulated to express connexin-43 by exposure to interferon-γ and tumour-necrosis factor. The infected cells were loaded with a dye such that monocytes that had obtained peptides through gap junctions were detectable. Only dye-containing monocytes were found to stimulate a cytotoxic-T-lymphocyte response.

These findings indicate a novel mechanism for cross-presentation that does not require release of intracellular antigens from cells. Interestingly, Langerhans cells and intestinal dendritic cells have many gap-junction contacts with surrounding cells, and monocytes can form gap junctions in response to 'danger' signals. In addition, gap junctions are inactivated in many tumour cells, indicating that this is another mechanism by which tumour cells might avoid detection. Apoptotic bodies can be cross-presented to CD8+ T cells, but in situations in which cells are prevented from undergoing apoptosis, cross-presentation could be achieved by GMIC.