We too have seen what we strongly suspect to be eosinophils staining
with anti-CD34 conjugates (class III HPCA2 PE). In our case it was
from atopic individuals whose blood contains elevated levels of CD34+
cells.
'Normal' CD34+ cell in peripheral blood form a discrete cluster on
CD45 vs side scatter analysis (see ISHAGE Guidelines for CD34+ cell
Enumeration, J. Hematotherapy 5:227-236, 1996). Symptomatic atopic
individuals generally exhibit increased numbers of these cells but the
cluster tends to form a continuous 'comet-tail' towards increased side
scatter and CD45 expression. These cells exhibit the light scatter
characteristics that you described for your 'mystery population'.
In the PB of asymptomatic atopic individuals, we often see an increase
in basophil/eosinophil progenitors (American Journal of Respiratory
Cell and Molecular Biology 15: 645-654, 1996.)
Although beyond topic, we have also seen CD34+ basophils in transient
leukemias of newborn Down Syndrome samples and more commonly in marrow
samples from CML patients.
Finally (and even further beyond topic), we have seen a marrow (from
what turned out to be an MDS patient) that contained nearly 15% CD34+
cells, half of which exhibited distinctly granular cytoplasm by
fluorescence microscopy. Many of these cells formed a 'comet tail' on
CD45 versus side scatter but in this case they virtually overlapped
the neutrophil population.
Aren't annecdotes wonderful?
Rob Sutherland