Markers of cell lineage, differentiation and activation.

The most widespread use of CD markers is in the determination of cell lineage and sublineage. For example, T cells are identified by the expression of CD3 (reviewed in this issue of CD corner). A mature T cell may belong to the T4 subset, in which case it will express CD4. Similarly, there are markers for other cell populations and sub-populations. Within the lineages, it is helpful to distinguish cells at different stages of differentiation and activation. Differentiation status is particularly useful in the diagnostic analysis of the lymphoid and myeloid malignancies, and in research on the haemopoietic system. Examples include markers for naïve or antigen-experienced cells (especially the CD45 isoforms) and molecules such as CALLA (CD9) found on B-lineage precursors, including B lineage acute lymphoblastic leukaemia. Activation status is especially interesting in studies of cell function. Activation markers include growth factor receptors such as CD25 (a component of the receptor for IL-2), and molecules who's cellular function is not fully understood, such as CD69 and CD98. These markers have revolutionised aspects of pathology and research, and the ease with which some cell populations can be identified has lead to some unrealised, and perhaps unrealistic, expectations. We expect to be able to identify T helper type 1 (TH1) and T helper type 2 (TH2) cells on the basis of a simple surface marker; we are frustrated by the lack of a single marker for all dendritic cells or for all NK cells; we are confused by the un-coordinated expression of different activation markers; we tend to over-interpret phenotype in some situations. To find solutions to these problems it is helpful to examine why the successful lineage markers work so well, and to reconsider our expectations. Lineage markers are the main focus in this commentary; the question of activation markers and markers of differentiation state will be considered in a separate paper.