CD's

SUSAN LECLAIR

SUSAN LECLAIR

Expression percentages are fairly easy to understand. The higher the number the more cells in a population show that receptor. So, for example, a test for insulin receptors should always come out as 100% since every cell should have these receptors.

CD22 acts to bring messages from the "outside" to the inside of the cell so that the cell can respond to a change in the environment. It also enhances adhesion of the cells. It should be found on mature B cells and pre-B cells. Once these cells have been activated to respond to an immune challenge, CD22 is supposed to be turned off. In CLL and NHL, it is not. CD22 has been seen

Somewhat simplistically - B cells need several different types of receptors. One set would be to recognize that an immune challenge is present. Another set would decide if a B cell response is appropriate. Another set would tell the cell to become reactive to this challenge. Another set would tell the cell to undergo mitotic division in order to build up a large number of cells that are capable of mounting an immune defense. Another set would determine if the defense was successful while another set decides that it is time to shut down active response and finally, several other sets provide memory. CD22 appears to be in the receptor set that tells the cells to undergo mitotic division and build a clone of cells. So an excess of CD22 would cause a cell to undergo "clonal expansion" for no reason.

The numbers of who has what are changing rapidly. This is due to the increased number of patients who are having flow cytometry done. Remember that it was only a decade or so ago that CLL and NHL were not even considered for cytometry testing. So as more and more patients are tested, the percentages change. For example, when Wintrobe first described the red cell indices (MCV, MCH, MCHC) in the 1930s, he used the values from approximately 200 adult males as his reference range. It was 86-94. It wasn't until the mid 1970's when automated instrumentation made these tests an integral part of the CBC that we got enough numbers from enough patients with different ages, genders, and locations to finally prove that these values were the same across gender and location but not across age. And the reference range got bigger moving to 80-100.

The assumptions for using anti-CD22 are the same as those for anti CD20 and anti-CD52 (rituximab and campath and lots of others being developed/trial-ed right now. The logic with immunologic therapy (Anti-CD22) is that by killing off those cells with CD22 expression, you will kill off lots of malignant cells without causing too much damage to other cells. Will the therapy kill off normal cells that are expressing CD22? Yes. The rationale here is that you can always replace them through normal processes but the damage to the malignant cells might be so complete that the abnormal clone will die completely.