Apollo is built on GraphQL, which despite its name is not a query language for graph databases. Facebook describes it as an application layer query language, and the easiest way I've found to think about it is a structured, strongly typed schema and query language that replaces or complements REST APIs.
What that means is that you can use Apollo together with TitanDB. GraphQL is the perfect technology to serve as a gateway in front of your various backend microservices or databases. This Medium post explains some of the concepts in more depth.