MONITORING THE CELLULAR COMPONENTS OF THE IMMUNE SYSTEM DURING CLINICAL TRIALS: A TRANSLATIONAL MEDICINE APPROACH - Flow Cytometry in Drug Discovery and Development - page 195

with 6-18 color capabilities have used additional markers to define these T-cell
subsets. The questions that arise now are how comparable are the data? Can results
from studies using one phenotype (CD45RA and CD27) be compared to others in
which a different phenotype was used (CD45RA and CCR7, or more advanced
phenotypes)?Would the trends be the same? Are we comparing apples and oranges or
merely tangerines and oranges? If a newer, more extensive, phenotype has been
described, should that new convention be adopted even though earlier studies were
conducted with a different phenotype? If so, can the data be compared?
A similar lack of definitive phenotypes exists for Th1 and Th2. Initial reports in the
literature phenotypically identified Th1 and Th2 on the basis of CD45RA, CXCR3,
and CCR4. More recently, Th1 and Th2 have been described as CCR7 , CCR5 þ ,
CXCR3 þ and CCR7 , CCR3 þ , CCR4 þ [36]. Many questions arise. Is the first
phenotype still legitimate? Are CD45RA and CCR7 interchangeable? Is it sufficient
to look at CXCR3 only without further stratifying into CCR5 positive cells? Can one
evaluate the phenotype directly from whole blood without the ex vivo step to induce
cytokine secretion? Does this approach have the advantage of possibly providing a
more accurate approach to assess the immune status in vivo?
At present, the phenotype for Treg is well established as CD4 þ , CD25bright, and
intracellular Foxp3 þ or CD4 þ , CD25bright, and CD127dim/- in cases where
intracellular staining is not feasible. This was not the case as recently as 4-5 years
ago, before Foxp3 was identified as the definitive additional marker for human Treg
and good-quality reagents were available. In those early days, enumeration of human
Treg by immunophenotyping without sorting and functional studies was not possible.
In such cases, being ahead of the curve is not to one's advantage.

9.6 SUMMARY

The translational medicine approach in drug development is still fairly new and the
benefit of this approach has yet to be demonstrated. To date, very few of the many
biomarkers evaluated in clinical trials have translated into useful disease-specific
diagnostic or prognostic biomarkers. Nonetheless, the enthusiasm for this approach
lies in its vast potential. Similarly, this is an exciting but challenging time in the
evolution of flow cytometry. We have technologically reached the point where we can
observe as many subsets as we have cellular markers leaving us with the conundrum
that the more information we gather, the more questions we raise. Thus, translating
basic scientific findings into useful cellular PD biomarkers and translating the
information back is an enormous, albeit intriguing, challenge.

ABBREVIATIONS

Ab antibody

BCR B-cell receptor

CD cluster of differentiation