property Items.ItemUnderline(Item as HITEM) as Boolean

Retrieves or sets a value that indicates whether the item should appear in underline.

TypeDescription
Item as HITEM A long expression that indicates the item's handle.
Boolean A boolean expression that indicates whether the item should appear in underline.

To change the underline attribute for a cell you can call CellUnderline property. 

The following sample underlines the selected item in the first group:

Private Sub ExplorerTree1_SelectionChanged(ByVal Group As EXPLORERTREELibCtl.IGroup)
    If (Group.Index = 0) Then
        With Group.Items
            If Not hOldItem = 0 Then
                .ItemUnderline(hOldItem) = False
            End If
            hOldItem = .SelectedItem()
            .ItemUnderline(hOldItem) = True
        End With
    End If
End Sub