one item in combobox is bold other are not bold - how to make it?

Question

Good day! I have a combobox in a silverliht application

<ComboBox  x:Name="cbCities" Width="500" Height="24"/>

and bind to it a list of items of City class. The items of City class have a property isCapital There is an only one or no any items with isCapital = true in the list I want to make the item with isCapital = true to be bold in the combobox, and other items not ot be bold. How can I do it?