Populate WPF listbox based on selection of another listbox in mvvm

I have a listbox that is bound to an observablecollection. The observable collection contains a list of string,This iis prsent in HelperClass(Say Class1) each with it's own observablecollection. What i want is to click an item in the first listbox and have it's list of things displayed in the second listbox. Can I do this in pure WPF?

I am Using MVVM.and All my logic is Present in helper class (Class1) both bservable collections

Now I want to Bind this data through ViewModel

How can I do this?