Cluster of differentiation / CD antigens are surface proteins that belong to several different classes, such as integrins, adhesion molecules, glycoproteins, and receptors. Different cell types have different CD antigens. Antibodies recognizing CD antigens are frequently used as an efficient tool in cell sorting and in identifying and characterizing various cell populations.
Several CD antigens are associated with mouse and human Embryonic stem cells (ESC). CD9 is known to be developmentally regulated in both mouse and human ES cells. The CD antigens associated with pluripotent human Embryonic stem cells are CD24, CD30, CD50, CD90, CD133, CD200, and CD326. However, CD30 is not always expressed on human ES cells. CD133 is also a hematopoietic stem cell marker. In addition, human Embryonic stem cell express markers such as CD90 and CD117. However, CD133 and CD96 are also expressed in some tumor stem cells. The expression of other cluster of differentiation antigens may not be strictly associated with the undifferentiated state, but reflect the presence of progenitor cells within a human ES cell culture, like as CD184 and CD87 , which have been postulated as lineage markers. The CD147 antigen reflects neither the differentiated or undifferentiated state, but has proved useful as a pan-human marker.
Hematopoietic stem cells (HSC) are characterized with the capacity for self-renewal as well as multi-lineage differentiation, maintaining the immune system and blood cell formation throughout life. HSCs are morphologically very similar to white blood cells. Thus, techniques designed for isolation and identification of HSCs are often dependent on the detection of cell surface markers and cluster of differentiation antigens. HSCs are commonly characterized by the absence of lineage-specific marker expression. Human Hematopoietic Stem Cells are determined as CD34, CD59, CD90, and CD38. Mouse Hematopoietic Stem Cells are considered CD34, CD90, and CD38. Major hematopoietic stem cell markers CD4, CD34, CD38, CD48, CD117, CD135, CD150, and CD244.
Mesenchymal stem cells (MSC) are multipotent stem cells (SC) derived from various sources. There are no specific markers to identify them; however, they are negative for hematopoietic cell markers like CD34 and express CD90, CD73 and CD105 on their surface. Their immunomodulatory effect on T-cells, B-cells, NK cells and dendritic cells and interactions with T-regulatory (CD4+) cells. MSCs can suppress the T lymphocyte proliferation induced by alloantigens, mitogens and anti-CD3 and anti-CD28 antibodies. MSCs have a similar effect on memory and naive T cells as well as CD4+ and CD8+ T cells and this suppressive effect does not require major histocompatibility complex (MHC) restriction. Cell inhibition is believed to be due to soluble/growth factors like IFN-γ, interleukin -1β, Transforming growth factor-β1 and hepatocyte growth factor in humans. Their immunomodulatory activity is believed to be through these growth factors and indoleamine 2,3-dioxygenase and prostaglandin E2. The secretion of human leucocyte antigen-G5 by MSCs is reported to be essential for their following effects: suppression of T-cell and NK cell function, shift of the allogeneic T-cell response to a T-helper type 2 cytokine profile and induction of CD4+CD25high forkhead box P3 (FoxP3+) regulatory T cells (Tregs).