Follow-ups to [HP89]?

In the introduction to their 1989 article, JME Hyland & AM Pitts, "The Theory of Constructions: Categorical Semantics and Topos-Theoretic Models", Contemp. Math. 92 (1989), 137 - 199, the authors say, "Clearly this paper is only a beginning." Can someone recommend follow-ups to this paper published in the intervening 15 years? Todd Wilson Department of Computer Science California State University, Fresno