Question about Messner and the Collectors (Spoilers)

How exactly was he working with them? My memories of ME2 aren't all that fresh anymore, but isn't Cerberus working against the collectors? How exactly did Messner end up working with the collectors? How did Cerberus get in touch with them and everything?

I just didn't really understand why the collectors would have agreed to work with Cerberus (like how they negotiated with them in the first place).

Also, if Messner's goal was to find information on the Collectors to help humanity against them, why was he working with them? I feel like I'm not understanding something here.