What makes for a complete QSO when using APRS?


I've just started using a new mode of communicating through satellites, APRS on the ISS. On two passes, now, I've successfully beaconed through it and saw the digipeat, but also received APRS messages on those as well. After a closer look, the message was directed at my callsign with something like "73 de A1BC" or "QSL de A1BC". Does that count as a full, two-way QSO? Normally, I would expect to need to acknowledge it, but I know APRS has built-in message acknowledgements and they did include 73/QSL. Grid squares and callsigns have been exchanged. Should I bother trying to acknowledge it at that point or is the QSO already complete?

- Loren