Abnormality in PMA treated THP1 cells - (May/06/2015 )

I treat THP1 cells with 5 ng/ml PMA for 48 hrs to differentiate. Recently, after 24hrs of PMA treatment I see black dots in the cytoplasm. These PMA treated cells are not properly responding to LPS. I have attached the pics for the problematic cells. Please suggest the possible causes for this abnormality and how can i troubleshoot it?

Looks like a contamination of your cells, you can also see some spots in the area around the cells.