The human bcl-6 gene, which is rearranged in about 30% of diffuse larg
e B-cell lymphomas (DLCL-B), encodes for a Kruppel-type zinc finger pr
otein of 706 amino acids. In order to investigate the expression of th
e bcl-6 gene at the protein level, two monoclonal antibodies (PG-B6a a
nd PG-B6p) directed against the human bcl-6 protein were generated by
immunizing Balb/c mice with a recombinant protein corresponding to the
amino-terminal region (amino acids 3-484) of bcl-6. PG-B6a (a = avian
) recognized the most conserved bcl-6 epitope (expressed in many anima
l species, including avian). PG-B6p (p = paraffin) reacted with an epi
tope of bcl-6 partially resistant to fixatives and detectable on micro
wave-heated paraffin sections. At immunocytochemistry, bcl-6 localized
in the nucleus with a microgranular or diffuse pattern. Strong nuclea
r expression of bcl-6 was mainly detected in normal germinal-center B-
cells, whereas mantle- and marginal-zone B cells, as well as plasma ce
lls and marrow B-cell precursors, did not express bcl-6. These immunoh
istological findings strongly suggest that bcl-6 may play a role as a
regulator of germinal-center related functions. All MoAbs stained neop
lastic cells of follicular lymphomas, DLCL-B, and Burkitt's lymphomas.
In DLCL-B, bcl-6 expression was independent of bcl-6 gene rearrangeme
nts and did not correlate with expression of other markers or the prol
iferation index. Among low-grade B-cell lymphomas, immunostaining for
bcl-6 proved useful for differentiating proliferation centers in B-CLL
(bcl-2+/bcl-6-) from trapped germinal centers in mantle-cell lymphoma
s (bcl-2-/bcl-6+). Strong nuclear positivity for bcl-6 was consistentl
y detected in tumor (L&H) cells of nodular, lymphocyte-predominant Hod
gkin's disease (NLPHD). These results further support the concept that
NLPHD is a histogenetically distinct (germinal-center derived) subtyp
e of HD. Notably, the nuclei of reactive CD3+/CD4+ T cells near to and
resetting around L&H cells in NLPHD were also strongly bcl-6+, but la
cked CD40 ligand (CD40L) expression. This staining pattern clearly dif
fered from that of classic HD, whose cellular background was made up o
f CD3+/CD4+ T cells showing the bcl-6-/CD40L+ phenotype. The above imm
unohistological findings suggest that (a) bcl-6 may play a role in reg
ulating B-cell differentiation step(s) occurring within germinal cente
rs; (b) deregulated bcl-6 expression caused by rearrangements may cont
ribute to B-lymphomagenesis; (c) bcl-6 is possibly involved in the pat
hogenesis of NLPHD.