Monocytes originate from myeloid progenitors in the bone marrow, circulate in the blood for up to three days, and then enter peripheral tissues where they differentiate into macrophages or dendritic cells .
Human monocytes can be divided into three subpopulations: classical, intermediate, and nonclassical. This subdivision is based on the expression levels of the lipopolysaccharide (LPS) coreceptor CD14 and the Fcγ receptor III CD16. Classical monocytes, the major population of monocytes, are CD14++CD16−. The minor population of CD16 expressing monocytes are further subdivided in intermediate monocytes (CD14++CD16+) and nonclassical monocytes (CD14+CD16++) . Recently, monocyte subpopulations have been extensively investigated. In addition to CD14 and CD16, also other markers, such as HLA-DR, CCR2 and CCR5, can distinguish the three human monocyte subpopulations [3, 4].
An imbalance in the relative proportion of CD16 expressing monocytes has been found in a variety of immune mediated diseases such as rheumatoid arthritis, diabetes, atherosclerosis, bacterial and HIV infections (reviewed by Ziegler-Heitbrock ). An increase in the relative number of CD16 expressing monocytes was found in newly diagnosed sarcoidosis patients, suggesting an activated state of the monocytes. This activated state might be an indicator for disease activity [6, 7]. These investigations have not discriminated between the two CD16 expressing monocyte subpopulations. Recently, it has been shown in patients with rheumatoid arthritis  and severe asthma  that intermediate monocytes are responsible for the increase in CD16 expressing monocytes. So far, this has not been investigated in patients with sarcoidosis.
Sarcoidosis is a systemic, granulomatous disease of unknown origin that primarily affects the lungs. It is a disease of all races and ethnic groups with varying incidence throughout the world [10–12]. The highest incidence of sarcoidosis in Europe has been reported from Sweden: 24 cases per 100,000 . In The Netherlands, the incidence of sarcoidosis is estimated to be 20 cases per 100,000 . In the United States, the incidence rate in black people is 36 cases per 100,000 compared to 11 cases per 100,000 in white people . The most important feature of sarcoidosis is the formation of non-caseating granulomas [10–12].
In the human lung, tumor necrosis factor (TNF) is mainly produced by alveolar macrophages and thought to play a major role in the development of the granulomatous inflammation. The release of TNF is increased in the lung of patients with pulmonary sarcoidosis [16, 17]. Therefore TNF is an attractive target for immunotherapy in sarcoidosis.
TNF exerts its function by binding to and signaling via two different receptors. TNFR1 (CD120a) is constitutively, but in low levels, expressed on nearly all nucleated cell types. TNFR2 (CD120b) is inducible and expressed by cells of myeloid lineage, peripheral T cells and alveolar lymphocytes and macrophages [18–23]. TNFR1 activates the caspase family, which induce cell death. TNFR2 signalling leads to NF-ĸB transcription and can induce proliferation, differentiation, cytokine production and even apoptosis. When TNFR1 and TNFR2 are co-expressed on the same cells, intracellular cross-talk between the receptors may occur . Both TNF receptors can be enzymatically cleaved from the cell surface and form soluble TNF receptors. Soluble TNF receptors can neutralize TNF and clear it from the circulation and act as a TNF antagonist, but can also prolong its bioactivity by binding TNF and thus stabilize the trimeric structure . Membrane-bound TNF binds more strongly to TNFR2 than soluble TNF does. It may be that TNFR2 only becomes fully activated with membrane-bound TNF .
There are a few reports of TNFR1 and TNFR2 expression on peripheral blood cells. Approximately 25% of the peripheral blood lymphocytes in healthy subjects is CD4+TNFR2+. Interestingly, in sarcoidosis the percentage of CD4+TNFR2+ cells is increased, especially in patients in remission or with stable disease (respectively 35% and 33%), which may indicate a role in down regulation of a cell-mediated immune response . FoxP3+ T reg cells expressed the highest levels of TNFR2 among subsets of human peripheral blood CD4+ T cells and were able to shed large amounts of sTNFR2, suggesting an important role for T reg cells in suppressing TNF . It is attractive to speculate about a similar role for one of the subpopulations of human monocytes. However, the expression of TNFR1 and TNFR2 on monocyte subpopulations has not been studied thus far.
The aim of the present study was to determine frequencies of the three monocyte subpopulations in peripheral blood of patients with sarcoidosis compared to healthy controls and the relative expression of TNFR1 and TNFR2 on these subpopulations.