Dear Hallie, 

cell line are the homogenous population of a single cells and have ability to divided continuously without any hormone or cytokine stimulation as long as you culture them in suitable medium. If cell line are monocytes they will keep on dividing into monocytes only. They can't differentiate into other type of cells. However, by supplementing growth regulators or cytokine you can make them to differentiate into Macrophages or dendritic cells. 

You can use a cell line like J774s or you can isolate mouse monocytes from mouse bone marrow and differentiate them when needed.

Indeed, J774.A1 are a frequently used Mouse cell line. However, I am not aware of a mouse line that can be differentiated into macrophages upon request as is the case for human THP-1 cells.

I have prepared an overview on mouse and human monocyte/macrophage lines which can be found here:

http://stefanlorkowski.de/macrophages_cell_lines.html

Please note that the overview has not been updated for some time.

Best regards

Stefan