ROCHESTER, New YorkControl of colon cancer may be
mediated in part by the patient’s immune system, suggesting that treatments
that enhance this innate capability could aid in reducing mortality. This is
the principal conclusion taken from work carried out by Alok Khorana, MD, and
his colleagues at the James P. Wilmot Cancer Center, University of Rochester
Medical Center, and presented at the American Association for Cancer Research
94th Annual Meeting (abstract 512).
The study involved immunocyto-chemical staining of
paraffin-embedded stage II and III primary colon cancer tumors from 131
patients, seen at the University of Rochester from 1990 to 1995, and focused
on vascular endothelial growth factor (VEGF), a protein that is overexpressed
in a variety of cancers, often with an associated increase in severity of
disease.
In addition to examining VEGF expression in the tumor
itself, the investigators also measured its appearance in the surrounding
tissue stroma, paying particular attention to its association with
tumor-associated macrophages (TAMs). Sections were also stained for the
antigens C68, a marker for macrophages, and CD3, a marker for T lymphocytes.
Immunostains were separately graded as the percentage of
cells stained with moderate or greater intensity according to the following
scale: negative, less than 2%; grade 1, 2% to 20%; grade 2, 20% to 50%; grade
3, greater than 50%. The degree of staining was further correlated with length
of survival after resection of the primary tumor. Data were analyzed using a
multivariate Cox proportional hazards model.
Study Results
Despite its frequent association with more severe disease,
VEGF expression in the tumor itself was not significantly associated with
survival. In contrast, there was a clear relationship between survival and
VEGF expression in TAMs; 42% of patients showed this expression pattern, and
median survival in this group was more than doubled, to 9.7 years vs 4.3 years
in the VEGF-negative TAMs group (P = .0065).
Thus, rather than being associated with reduced survival,
increased VEGF expression in one subset of tumor-associated cells actually
predicted a more positive outcome. Confirmation that the stromal cells that
expressed VEGF were indeed macrophages came from co-staining for the
macrophage marker CD68; the great majority of VEGF-expressing cells were
macrophages, with the remainder being fibroblasts. The VEGF-expressing TAMs
comprised only a subset of the macrophages in the stroma, however, and in
multivariate analysis, there was no association between survival and CD68
expression alone. It was accordingly inferred that the improved survival was a
function of the presence of the VEGF-expressing TAMs subgroup, rather than of
all macrophages.
One clue as to the possible mechanism mediating the link
between increased survival and the presence of VEGF-expressing TAMs came from
the examination of T-lymphocyte infiltration, as determined from the pattern
of CD3 immunostaining. Only those tumors with VEGF-expressing TAMs showed
significant infiltration of T cells (n = 10; P = .0003). Thus, it is
possible that VEGF expression by TAMs is associated with mobilization of the T
cells that are involved in a host immune response against the tumor.
"At present, we have only a correlation between VEGF-expressing
TAMs and two other parameters, survival and T-cell infiltration, so that
suggestions as to underlying mechanisms must still be regarded as
speculative," he said. One possibility is that the macrophages are acting
as antigen-presenting cells, and that T cells have been mobilized to the tumor
in response to specific tumor antigens displayed by the macrophages.
"While the correlation with VEGF expression seems clear, it remains a
puzzle as to what role, if any, their VEGF expression plays in this
process," he said. "It may be, in fact, that it is only acting as a
surrogate marker of the activation of immune-related processes in given
cells."
In this connection, he said, it has recently been reported
that the transcription factor HIf-1α is essential for the activation of
cellular inflammatory pathways (Cramer et al: Cell 112:645-657, 2003).
"Since HIf-1a also upregulates VEGF expression, our correlations with
increased patient survival may reflect the impact of overall activation of
this subset of macrophages, not their VEGF expression as such," he said.
This could also explain why VEGF production by the tumor
itself is not protective. However, he noted that VEGF is present in several
isoforms, and the antibody used in the current study does not discriminate
among them. It remains to be seen whether the macrophage-associated VEGF and
the tumor forms are identical or different, he said.
"Obviously," Dr. Khorana said, "there is still a substantial
amount of work to be done to understand these phenomena. Nonetheless, we
believe that the most likely explanation for the improved survival associated
with VEGF-expressing TAMs is an involvement of the host immune response in
controlling the tumor. The implications are that further modulation of the
immune response through therapeutic intervention may improve the prognosis for
colon cancer." Some of this work has since been published (Khorana AA et
al: Cancer 97:960-968, 2003).
