Receptor-ligand specific labeling of immune cell interactions

This week journal Nature published new study in immunology that could be best described as a method paper. I personally don't understand the value of this paper been in Nature. Only positive characteristic I see in it is that experiments reported are done in a classical, cellular immunology "fashion" and very easy to follow and understand. Lets examine.

The new method, called LIPSTIC, which this study reported is about labeling receptor-ligand pair with naturally occurring enzyme, the Staphylococcus aureus transpeptidase sortase A (SrtA). SrtA appears to covalently transfers a substrate containing motif ‘LPXTG’ to a nearby oligoglycine (G5). Basically, receptor is genetically fused with SrtA and ligand is fused with G5 and when they interact, SrtA catalyses the transfer of the substrate onto the G5-tagged receptor. This transfer can be visualized by attaching to the substrate small labels such as biotin or a fluorophore.  

First the authors showed in vitro and ex vivo that the substrate transfer was specific to fused receptor-ligand pair and did not occur when enzyme was inactive or fused to unrelated receptor. Most of the reported experiments were done using CD40L/CD40 pair (CD40 signaling is relevant for DCs and CD8 T cell activation by CD40L expressing CD4 T cells).

Receptor-ligand specificity was maintained in vivo as well. Because CD40L upregulation on CD4 T cells depends on antigen-specific interactions, substrate transfer were restricted to those CD40+ DCs that were pulsed with cognate peptide (OVA).

However, this antigen-specific CD40L/CD40 interaction was maintained only for initial 10h-24h period. When OVA-specific T cells were left with DCs for longer period (48h) then even DCs pulsed with irrelevant peptide (LCMV peptide) got labeled with substrate. 

It is not clear what are the biological consequences of such non-specific T/DCs interactions. Are these two different DCs activated antigen or non-antigen specific manner somehow different with the regard of activation of CD8 T cells? We don't know. The problem with such model is that if activated CD4 T cells expressing CD40L can interact with CD40+ DCs and activate it (license it, to use polly matzinger's words) then we should expect that body should harbor only activated DCs because body constantly contains some number of CD40L+ activated CD4 T cells specific for all kind of antigens. It is possible high number of T cells transferred in these experiments created an artificial outcome.

In summary, this study showed new method how to label receptor-ligand pair in vivo. However, overall relevance of this method is not clear at present.

posted by David Usharauli