It is a very difficult task to cultivate monocytes without differentiation for a long period of time (more than 24-48 hours). Even if you left cells without stimuli (M-CSF, GM-CSF, IL-4 e.t.c.), monocytes will adhere and differentiate over few days. However, the most common culture media for monocytes are RPMI 1640 and DMEM + 10% FBS or FCS.

I think that one of the possible solutions to overcome the problem with differentiation is to maintain monocytes under conditions that more closely mimic those in the circulation (e.g. on an orbital shaker).

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3868191/

I recommend also reading:

“Human monocytes cultured in suspension in serum-free medium”

https://patents.google.com/patent/EP0205387A2/en

There is almost no way to keep a culture in suspension if there is no movement. Everything settles with gravity. If you culture on any plastic, monocytes will stick and become activated. Glass might be a better container, and you do need some movement to keep the cells from settling.

Is there a reason you are culturing monocytes in vitro before LPS stimulation?

Our lab has been trying to set up a monocyte culture for years now and we find that it is possible to keep them alive and unactivated for up to 8hrs only. Nonclassicals will start to die after that time period and Classicals will differentiate into macrophages. We use 10% IgG-depleted FBS in the cell culture medium (RPMI with 2mM L-glut + 1mM NaPyruvate + 1x nonessential amino acids + 2uM 2-mercaptoethanol) and always culture PBMCs (they tend to like having their friends around).