in the steady state, immature myeloid cells lack suppressive activity and are present in the bone marrow, but not in secondary lymphoid organs. In healthy individuals, immature myeloid cells (IMCs) generated in bone marrow quickly differentiate into mature granulocytes, macrophages or dendritic cells (DCs). In pathological conditions such as cancer, various infectious diseases, sepsis, trauma, bone marrow transplantation or some autoimmune disorders, a partial block in the differentiation of IMCs into mature myeloid cells results in an expansion of this population. 

You can find more info at 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2828349/

Many thanks, Samar.

I agree that the IMCs in bone marrow are non-suppressive. But in healthy donors, there are still a few MDSCs (0.5%-1%) present in the blood circulation. What I am thinking is that the differentiation into MDSC from IMC is not completely blocked in steady state as there is always a low-level inflammation in our immune system. That is why we can still detect them. But so far I didn't find any papers talking about MDSCs from healthy individuals.  I think it is because of the very low frequency in steady state, making it hard to sort them and do functional study. Do you have any ideas about this or have you ever read some papers talking about this?  Thank you :) Ang

You can read this

http://www.ncbi.nlm.nih.gov/pubmed/23248262

there are many parameters which might affect their function in the coculture with T cells. GM-CSF or IFN-gamma sectered by stimulated T cells might induce some suppressive potential in these immature cells. There will be bidirectional crosstalk between these immature cells and Stimulated T cells.

Many thanks Asif!  The figure 2 inside is what I has been looking for! Cheers!