Creating instance of T without direct access to some type parameters

I have two generic classes: A and B. From B I have to create instances of A. But the main problem is I don't know how to pass information about collection items type to type parameters list in A constructor: new A</*What must be written here?*/>().

    // There we must know what T type parameter represents.
    public class A<TItem>
    {
        public void Method(IEnumerable<TItem> sequence)
        {
        }
    }

    // TObject can be everyone type. It is not contrained.
    public class B<TObject>
    {
        public void Method(TObject obj)
        {
            // Assume that we ensured that obj is IEnumerable<>, but how to create instance of A?
            // May be there is another way to solve this problem...
        }
    }