The media recommended for these cells is Leibovitz's L-15 medium which does not contain sodium bicarbonate to act as a buffer for the carbonic acid that would normally form in the presence of CO2.
Furthermore, if the cells are cultured in Leibovitz's medium and grown in the presence of CO2, it will cause reduced cell viability. If the cells are grown in other types of media such as DMEM, this is not a concern.