I'm working on issues exchange between Integrity and Dante (via Integrity Integration Platform, but it is not so relevant now). There is a "Protocols" feature in Dante, for examlple, it looks like this:
<entry name="supplier project manager" text="issue accepted, will be fixed in the desired release" timeStamp="2013-11-22T12:08:52+00:00"/>
<entry name="supplier exchangeUser" text="issue taken from customer" timeStamp="2013-11-21T12:13:53+00:00"/>
in addition to Thierry's idea you could either use the item's History log, if don't to be limited to text based discussions; or you could choose to utilize the audit log features provides by the server.
Some deeper information about your use-case might be helpful here.