2 related combo in DataGrid WPF MVVM

Dears,

i want to know how can i implement 2 related comboboxes in a datagrid in mvvm pattern,something like that lets say i have a combobox binded to countries and another is binded to cities,i want to cities comboxbox in a selectedrow of a grid to be filterd according to the country selected in the countries combobox and if i changed the country selected of a selected row the cities combobox is filled with the cities of the selected country.

kindly provide an example in mvvm pattern

ur fast response is highly appreciated

thanks