**Executive Summary of the Tumor Microenvironment Think Tank**

The microenvironment in which a tumor originates plays a critical role
in tumor initiation and progression, and may be an important factor in
developing therapeutic approaches. The tumor microenvironment, or
stroma, influences the growth of the tumor and its ability to progress
and metastasize. It also can limit the access of therapeutics to the
tumor, alter drug metabolism and contribute to the development of drug
resistance. Because of their role in all the stages of tumor
development, stromal elements represent attractive therapeutic targets.
Manipulating host-tumor interactions may be important in preventing or
reverting malignant conversion, and re-establishing normal control
mechanisms.

Despite the importance of tumor-stromal interactions, there is a limited
understanding of the stromal composition, and of the complex
relationship between the tumor cells and the surrounding host cells. It
is now acknowledged that tumor cells and their stroma co-evolve during
tumorigenesis and progression. Stroma consists of cells, extracellular
matrix and extracellular molecules. Among the identified cells are
fibroblasts, glial cells, epithelial cells, adipocytes, inflammatory
cells, immunocytes, and vascular cells. However, the precise nature of
the cells that comprise normal stroma, how these cells or newly
recruited cells are altered during tumor progression, and how they
reciprocally influence tumor initiation and progression are poorly
understood.

As these salient and outstanding questions are addressed, it will be
possible to begin to develop complementary therapeutic strategies
targeted at both the microenvironment and the tumor. Among the
approaches envisioned to target the tumor microenvironment are the
development of drugs that induce apoptosis or inhibit the function of
the stromal cells, or the factors secreted by stroma that are required
for tumor progression and metastasis. It is expected that understanding
the tumor microenvironment will lead to the development of better
diagnostic tests and/or improved therapeutic strategies. Finally, it may
be possible to develop strategies to prevent the development of tumors
based on our understanding of alterations in the microenvironment that
enable tumor development.

The research priorities listed by the think tank include: a better
understanding of tumor microenvironment, and the identification and
characterization of the signatures of seemingly normal cells within the
tumor microenvironment and signatures that reflect changes that occur as
cancer cells interact with the host microenvironment. Achieving these
goals can be expedited by encouraging interdisciplinary research teams
and multi-institutional collaborations. Similarly, advances in
technologies will be critical for progress. Among the technologies that
have been identified as critical are: 1) novel in vitro 3D matrix
reconstitution and organotypic models, and animal models; 2) techniques,
such as laser capture microscopy, for the isolation and characterization
of stromal cells; 3) the discovery of novel stromal markers through
molecular profiling and their application for the development of
reagents for in vivo imaging to visualize tumor-host interactions. These
technologies will provide the tools for a better understanding of the
tumor microenvironment and for the development of tissue- or
cell-specific targeting agents. Successful approaches to respond these
needs can have a dramatic effect on making the tumor microenvironment
\"hostile to the tumor\", thereby transforming cancer into a \"chronic,
but benign\" disease.

**Introduction**

The intrinsic and extrinsic influences that transform a normal
epithelial cell into a malignant cell are very complex. Enormous
advances have been made over the last several decades in identifying the
molecular and genetic changes of cancer cells and the pathogenesis of
neoplasia. This has led to the identification of oncogenes and tumor
suppressor genes and associated signaling mechanisms by which they
modulate growth, survival and proliferation. These studies have
generated novel therapeutic reagents such as Tamoxifen, Herceptin and
Gleevec.

Stromal influence on epithelia cells begins at fertilization and
continues during adulthood, wherein the microenvironment controls normal
development and homeostasis. For example, macrophage association with
the developing mammary gland is critical during development and CSF-1 or
CSF-1R null mice (devoid of macrophage) have defective mammary glands.

Research on tumor-host interactions collectively suggests that (a)
tumors are not autonomous masses of cells but function as organs
composed of many interdependent cell types that contribute to tumor
development and metastasis, and (b) the interactions between the tumors,
the extracellular matrix (ECM) and stromal cells is bidirectional and
dynamic; stromal cells include fibroblasts, adipocytes, glial cells,
smooth muscle cells, and resident and recruited vascular and immune
cells.

The tumor microenvironment and the malignant cells themselves constitute
the tumor entity that clinicians confront when treating cancer patients.
The cell-cell and cell-matrix interactions that influence the behavior
of cancer cells are targets with as much potential for the development
of effective therapies as the tumor cells themselves. The
microenvironment can exert both positive and negative influences on
tumor cells. Stromal cells can also impart stimulatory and growth
inhibitory effects on tumor cells, e.g., the malignant potential of
teratocarcinoma cells can be restrained during embryonic development
resulting in cancer-free adult mice. Similarly, attenuation of β1
integrin (laminin receptor), EGFR or MAPK activation in highly
aggressive human breast cancer cells results in a reversion of the
aggressive phenotype.

The cancer cell is absolutely dependent on the stroma for its
proliferation, progression, and metastasis; examples include the role of
inflammatory cells (via cytokine and protease secretion) in tumor cell
proliferation, angiogenesis, invasion and metastasis; the interaction of
host immune cells with the vasculature; the interaction of tumor cells
with the angiogenic endothelial cells, and the role of lymphangiogenesis
during metastasis to the regional lymph nodes. Stromal cells can also
influence organ-specific metastasis as evidenced by the role of
stromal-derived cytokines and growth factors (e.g., PTHrP, CXCR4, SDF1,
TGF B and RANKL) in breast and prostate cancer and multiple myeloma
metastasis to bone. Finally, interaction of bone marrow stromal cells
with multiple myeloma cells has been shown to contribute to the
development of drug resistance.

Stroma can be targeted for therapy. Recent successes (a) in patients
with multiple myeloma, where bone marrow stroma was targeted using
proteasome inhibitor to attenuate bone metastasis, and (b) the
development of anti-angiogenic drugs (Avastin and Thalidomide) which
target the endothelial cells illustrate progress towards this goal.

The two major goals are: to obtain information about the
microenvironment that would facilitate the diagnosis, prevention or
treatment of cancer, and translating this information into useful
clinical applications. These broad goals can be achieved through the
following specific objectives: (1) identify the key components of the
tumor microenvironment and define how these are altered during tumor
development. Identify the stromal compartment of normal tissues and
compare how these are altered in carcinogenesis. (2) Determine which
alterations in the tumor microenvironment that are critical for the
development, progression and metastasis; elucidate the mechanism
responsible for induction of these changes. (3) Identify tumor cell stem
cells and characterize the interactions between stromal cells and tumor
stem cells, as well as the role of tumor stem cells in metastasis. (4)
Develop therapeutic strategies to target the tumor microenvironment and
interfere with site-specific metastasis by the (a) development of drugs
that induce death or inhibit the function of the stromal cells that are
required for progression; (b) development of reagents to target specific
factors produced by stromal cells that are responsible for progression,
and (c) blocking the induction of stromal factors responsible for tumor
cell survival and proliferation. (5) Develop diagnostic tests to predict
outcome and/or design treatment. (6) Develop strategies to prevent the
development of tumors based on an understanding of the alterations in
the microenvironment essential for tumor development.

These issues can be best addressed with the availability of novel
technologies and model systems. Thus the development of novel in vitro
3-dimensional matrix reconstitution and organotypic models or animal
models, isolation of stromal cells from normal and tumor cells using
techniques such as Laser Capture Microdissection will aid in the
identification of stromal markers through molecular profiling
technologies. The availability of stromal markers will facilitate better
reagent development for in vivo imaging to visualize tumor-host
interactions as tumor cells invade and metastasize. The availability of
stromal markers will also expedite the generation of reagents for
tissue- or cell-specific targeting.

**Significance**

Critical stromal elements of the tumor are attractive targets for
prevention, because they have maximal influence over tumor cells in the
early stages of tumor development. As targets for therapy, they are less
likely to be genetically unstable than tumor cells and thus less likely
to develop drug resistance. Manipulating host-tumor interactions has the
potential of reverting the malignant phenotype and establishing normal
control mechanisms. Eventually, the desired goal is to reduce or
eliminate metastasis-associated morbidity and transform cancer
metastasis in to a chronic but benign disease.

**Specific Recommendations for the NCI:**

1\. **Establish an Interdisciplinary Tumor Microenvironment Network**
that includes pathologists, cancer biologists, cell biologists,
oncologists, engineers, physicists, bioinformatics experts and industry
representatives. Such a group will facilitate the study of normal and
malignant tissue microenvironments. This can be accomplished by both
centralized resources and widespread, varied funding opportunities.

2\. **Encourage study of the normal tissue microenvironment as a
prerequisite for understanding the microenvironments of wounded tissues
and tumor tissues**

> Establish and support a repository of normal stromal cells and matrix
> molecules to facilitate this research
>
> Develop additional and improved 3-dimensional reconstitution and
> organotypic models that will permit the in vitro study of
> microenvironmental elements

Key issues in understanding normal stroma are

-   What % of tumor is stroma?

-   What are the various cells in the stroma?

-   What are the matrix molecules?

-   How does it change over time and with tumor progression?

-   Genomic and proteomic analysis of tumor stroma and involvement of

> bioinformaticists

3\. **Develop a better understanding of tumor dependence on stroma**

-   Since experimental manipulation of stromal components in vivo has
    profound effects on tumor growth, progression to metastasis and
    immune response, there is a need for selective agents that target
    specific cellular and molecular stromal elements, and inhibit tumor
    progression.

4**. Delineate the role of the microenvironment in tumor progression
--key issues**

-   Do tumor promoters affect the stroma and facilitate metastasis?

-   Are there differences between the microenvironment of a primary
    tumor as compared to secondary sites?

-   Does the stromal compartment contribute to the "metastatic
    signature" of tumors?

-   Are some 'stromal' cells really tumor cells in disguise?

-   Can the microenvironment of tumors be effectively neutralized to
    inhibit progression?

-   Is organ metastasis achieved solely via hematogenous spread, or do
    transiting cells initially exit via lymphatics and subsequently
    spread hematogenously?

-   What is the role of inflammatory cells during invasion, migration,
    tumor growth and metastasis? To achieve this it will be necessary to
    study inflammation *[in vivo]{.underline}*, in relevant
    immune-competent (mouse) models of progression (transgenic or
    syngeneic xenograft)

-   Do stromal elements interact or modulate the tumor stem cell 'niche'
    and do these interactions change as tumor progress and metastasize?

5\. **Stromal Genetics -- key issues**

-   What is the role of host genetics in influencing the stromal
    formation?

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-   Does stromal composition differ in different inbred mouse strains?

-   Do epigenetic changes influence stromal composition?

6\. **Encourage the use of existing technologies for visualizing the
components of the stroma at the level of individual cells and
molecules**.

-   Expand access to multiphoton microscopy and deconvolution microscopy
    to study the microenvironment.

-   Develop novel imaging technologies to study tumor microenvironment;
    recruit/collaborate with engineers and physicists who can apply the
    most current imaging technologies to the study of the normal and
    tumor microenvironment.

7\. **Translation of basic knowledge to human disease**

-   Discover and validate therapeutic targets derived from the tumor
    microenvironment

-   Encourage mouse and human research to improve our chances of
    discovering useful targets.

8\. **Role of NCI**

-   Establish a network of interconnected, multidisciplinary
    investigators and collaborative groups to work together on
    understanding the tumor microenvironment, facilitated by both
    centralized resources and widespread, varied funding opportunities.

    Such an infrastructure should

-   Provide centralized administrative support.

-   Provide and maintain repositories

-   Produce and distribute reagents

-   Provide core facilities

-   Facilitate interdisciplinary collaborations.

-   Train scientists

    What kind of technologies should NCI support?

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-   Animal models / fluorescent markers

-   Imaging (multiphoton) at one micron resolution

-   Spectral deconvolution microscopy.

-   Selection of live cells from tumors

-   3D matrix reconstitution

-   Organotypic models

    Where would such facilities reside?

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-   In individual labs

-   In "Centers of Excellence"

-   In central or decentralized cores

-   At meetings and conferences

-   At courses on the tumor microenvironment

9\. **Establish and encourage a Systems approach** to the study of the
tumor microenvironment

10**. Encourage interactions with other NCI-supported multidisciplinary
groups** such as EDRN, MMHCC, SPORES, and Cooperative Groups.

11\. **Encourage interactions with other ICs** with similar interests
(NIEHS, NIDDK, NHLBI, NIAMD, and NIDCR)