Smarty does not display php variable

From a send user and password form.

$usuario = new usuario();
$u = $usuario->login($_POST['user'], $_POST['clave']);
$_SESSION['USUARIO_ACTUAL'] = serialize($u);
$tpl = new Plantilla();
$tpl->assign('VARIABLE', unserialize($_SESSION['USUARIO_ACTUAL']));
$tpl->display("variable.tpl.php");

The login class:

function login($user, $pass){

$query = "SELECT usuario, clave, acceso FROM users WHERE usuario = :usuario_in AND clave = :clave_in";

$BD = new ConexionDB();
$recordSet = $BD->prepare($query);
$recordSet->bindParam('usuario_in', $user, PDO::PARAM_STR);
$recordSet->bindParam('clave_in', $pass, PDO::PARAM_STR);
$recordSet->execute();

while ($fila = $recordSet->fetch(PDO::FETCH_ASSOC)) {
$usuario = new UsuarioVO($fila['usuario'], $fila['clave'], $fila['acceso']);
}
return $usuario;
}

And in the view I want to visualize the user:

{$VARIABLE->usuario}

But it shows me: Notice: Trying to get property of non-object in smarty compiler Call Stack

By doing print_r ($ u) it shows me:

UsuarioVO Object ( [usuario] => ADMINIS [clave] => tecsup [acceso] => 0 )