The tradition of HLDA Workshops
The process of categorizing the antigenic molecules and epitopes associated with human white cells, via the collaborative study of mAb, dates back to the early 1980s, when the first HLDA (Human Leukocyte Differentiation Antigen) Workshop was held in Paris. This initial meeting listed only 15 agreed molecular entities, but it created an internationally agreed basis for the nomenclature of leukocyte molecules (the CD scheme), and also provided a forum for reporting studies on their function and practical relevance. A further six HLDA meetings have been held since the first Paris meeting. The most recent of these (`HLDA7') took place last year in Harrogate, UK and the proceedings of the meeting will be published later this year (Leukocyte Typing VII, Oxford University Press).
The aims and approaches of the 7th HLDA Workshop
The limitations of `blind' antibody screening
It was apparent at the previous meeting, HLDA6, held in Kobe, Japan in 1996, that the technique of detecting molecular entities by screening coded panels of mAb against human cells was becoming obsolescent. Antibodies to the most immunogenic molecules had already been produced and fewer laboratories than in the early days were prepared to devote resources to raising new antibodies, since the probability of finding novel reagents becomes ever less likely. In consequence, many antibodies in the 6th Workshop were reagents (submitted by laboratories that were not equipped to characterize them) that proved to be of known specificity.
Selection of antibodies
With these considerations in mind, the 7th Workshop adopted a different approach: instead of screening poorly characterized antibodies, reagents were selected (and actively solicited) for which at least some molecular data were already available. A substantial number of mAb reactive with leukocyte-associated molecules exist that do not meet the traditional criterion for establishing a new CD specificity (i.e. the existence of at least two independent antibodies of the same specificity). This rule dates from the first HLDA Workshop two decades ago: since that time biochemical and molecular biological techniques for characterizing the targets of new antibodies have come to be widely used. Consequently, it is now considered appropriate to establish a CD designation for a molecule if its gene has been cloned and at least one specific mAb has been studied in the Workshop.
New Workshop Sections
Four new Sections were introduced in the 7th HLDA Workshop to add to the traditional list from past meetings, i.e. `Dendritic Cells', `Stem/Progenitor Cells', `Erythroid Cells' and `Carbohydrate Structures'. Although it has been recognized for many years that mAb reactive with human leukocytes can be specific for carbohydrate epitopes (e.g. the carbohydrate CD category CD15 was identified at the first Workshop), they had not received specific attention in any Workshop. The inclusion of erythroid molecules, although it may seem out of place in a `Leukocyte Workshop', was justified by the number of molecules shared between white and red cells (e.g. cytokine receptors) that hint at unexplored functions of red cells.
The yield of new CD specificities in the 7th HLDA Workshop
This more active approach to the identification of new CD specificities represented a break with tradition, but the results justified the new approach, since a total of well over 80 new entities were added to the list of CD specificities. This compares favorably with previous Workshops (an average of <30 CD specificities per Workshop) and it also largely avoided the laborious screening in multiple laboratories of antibodies that prove to be directed against known CD molecules.
Tables 1 and 2 list the new specificities established at the 7th Workshop. Full details will be found in Leukocyte Typing VII, and molecular, functional and other data can be found for many of these new specificities on the PROW (Protein Reviews on the Web) website (http://www.ncbi.nlm.nih.gov/prow/).
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Finally, it remains to be established how the 8th and subsequent HLDA Workshops should deal with lineage- or stage-restricted leukocyte molecules that are localized within the cell cytoplasm (or nucleus). Given the importance of many of these molecules in signaling pathways initiated via known surface CD molecules, their identification and study is an inevitable extension of the work of the first seven HLDA Workshops. Whether or not a new `intracellular CD' categorization scheme is devised for such molecules, they are of interest for many laboratories interested in human hematopoietic cells and their study will be among the aims of the next Workshop.
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