Automorphisms of the totally isotropic subspace inclusion graph of unitary spaces

Let $F_{q^2}$ be a finite field of $q^2$ elements, where $q$ is a power of a prime and ${\mathbb{U}}_n$ an $n$-dimensional unitary space over $F_{q^2}$. The inclusion graph of the totally isotropic subspace of ${\mathbb{U}}_n$, written as $\mathcal{I}n({\mathbb{U}}_n)$, is a graph which has all totally isotropic subspaces of ${\mathbb{U}}_n$ as its vertices and two distinct, totally isotropic subspaces $U_1$, $U_2$ of ${\mathbb{U}}_n$ which are adjacent if and only if $U_1\subset U_2$ or $U_2\subset U_1$. In this paper, all automorphisms of $\mathcal{I}n({\mathbb{U}}_n)$ are determined definitely.