High Grade, Large Cell Lymphomas

I was wondering if anyone could shed some light on why we are having trouble
identifying high grade, large cell lymphomas by Flow.  The Pathologists have
no trouble diagnosing these by morphology and immunohistochemical stains
performed in Histology, but the Flow results have been unable to pick up the
aberrant cells and support their findings.  We were blaming this on our
method of processing which includes using a closed system tissue grinder and
thought that these cells might have been too fragile to survive this step
because we were not even seeing large cells present.  Then last week we had
an FNA (which does not get exposed to the tissue grinding process) which
contained mostly very large cells that only stained with CD45 (weaker than
the normal lymphocytes), CD19 (negative to weak on APC and totally negative
on PerCP), and CD22 (negative to weak).  Morphologically these cells looked
like a high grade, large cell lymphoma and the CD20 was obviously positive
by immunohistochemical stains. 

The differences that I have found between the antibodies used my both
methods are the clone and isotype.  The antibodies that we use for Flow are
the clone L27 and the isotype IgG1.  The antibodies used for the
immunohistochemical stains are the clone L26 and isotype IgG2a.  Both are
monoclonal and mouse anti-human antibodies.

Has anyone else found this same problem in their institutions or have any
advice?

Thanks in advance, 
Brenda Rabeno, MT (ASCP)
Flow Cytometry/Tissue Procurement Laboratory
brabeno@christianacare.org