Understanding Immunoevasion

The microenvironment of a malignant tumor is widely understood to be biochemically complex. According to the American Association for Cancer Research:

“What is in the tumor and microenvironment? There are normal epithelial cells—those that are not cancer yet. There are fibroblasts and the cells that make up the blood vessels. There are the infiltrating immune cells that come from the bloodstream. There are structural components comprised of proteins—little strands of fibers that hold our cells together—which we call the extracellular matrix because it’s outside of the cell. Then there are lots of molecules…special kinds of growth factors called chemokines and cytokines, which are chemical activators and cellular activators.”

Converting scientific insight into effective clinical practice involves reducing the virtually infinite complexity of a particular biochemical system down to the most significant processes, and determining how best to manipulate those processes to reestablish normal system function. Despite the complexity of the tumor microenvironment, a relatively small set of molecules are responsible for immunoevasion, upon which the etiology of solid tumor cancer depends. OncoPherese employs the following understanding of this etiology – confirmed in various studies and described in detail in various publications:

1. Cell lines derived from a variety of tumors overproduce and shed the receptors for the cytokines TNF-alpha and Lymphotoxin (previously known as TNF-beta) spontaneously in tissue culture
2. Levels of two types of soluble shed receptors for TNF – sTNF-R1 and sTNF-R2 – are especially elevated in the sera of tumor-bearing patients with elevations being greatest in advanced disease, returning to normal during remission
3. Patient survival rates vary inversely with levels of soluble TNF receptors in blood; and
4. A variety of tumor types respond immunologically to the removal of soluble inhibitors to TNF (sTNF-Rs), leading to inflammation and ultimately their destruction in many cases.

Given that the cytokines TNF (formerly TNF-α, now just TNF) and Lymphotoxin (formerly TNF-β, now LT) appear to be neutralized by their respective shed receptors within the tumor micro-environment, and that this process is causally implicated in immunoevasion, it’s worthwhile to examine these cytokines and their receptors more closely.

Key Cytokines: Tumor Necrosis Factor (TNF) & Lymphotoxin

Tumor Necrosis Factor and Lymphotoxin are two interrelated proteins that, in native form, are homotrimers of 17 and 17.5 kD peptides, respectively. Their genes are located in tandem within the major histocompatibility complex of mammals.

TNF is primarily produced as a 212-amino acid-long type II transmembrane protein; from this membrane-integrated form the soluble homotrimeric cytokine (sTNF) is released via proteolytic cleavage by the metalloprotease TNF alpha converting enzyme (TACE, also called ADAM17). Both the secreted and the membrane bound forms of TNF are biologically active.

While lymphotoxin is predominantly produced by lymphocytes, TNF is produced by macrophages, lymphocytes and other cells in selected situations. These cytokines have many effects in vitro that depend on the target cell such as growth inhibition or lysis of transformed or malignant cells – hence the name “tumor necrosis factor.” In vitro and in vivo, TNF can activate phagocytic cells, up-regulate various cell surface proteins – particularly growth factors – and control the development and expression of cell-mediated anti-tumor responses. These are the responses seen in tissues and the tumor microenvironment. Killer mononuclear cells deliver these potent cytokines directly to the diseased cell and kill it. These are the “swords” that the immune system uses to do its killing.

Tumor specific necrosis by TNF/LT has evoked interest for potential use as anti-cancer agents. However, clinical trials to date that have focused on parenteral administration of these molecules so as to achieve supraphysiologic concentrations in blood has at best limited clinical efficacy and unacceptable toxicity – as would be anticipated now that we more clearly understand the physiology of these cytokines. When delivered without immune mediation directly into the vascular system by intravenous injection, these cytokines cause a spectrum of reactions, including necrosis of tumors but also leukocytosis and systemic inflammation, cachexia and shock. The precise biologic effect of these cytokines differs depending on the receptor cell type and location, which cannot be controlled when delivered systemically.

Key Receptors: TNF-R1 & TNF-R2

There are two distinct cell surface TNF receptors (TNF-Rs). They have been sequenced, cloned and designated: TNF-receptor 1 (TNF-R1, TNF-BP1, Type B, 55 kD or HTR antigen) and TNF-receptor 2 (TNF-R2, TNF-BP II, Type A, 75 kD or UTR antigen) with molecular weights of 55 and 75 kD respectively. TNF-R1 is expressed in most tissues, and can be fully activated by both the membrane-bound and soluble trimeric forms of TNF, whereas TNF-R2 is found only in cells of the immune system, and responds to the membrane-bound form of the TNF homotrimer.

Signaling pathway of TNF-R1. Dashed lines represent multiple steps.
Signaling pathway of TNF-R1. Dashed lines represent multiple steps.
Both TNF-R1 and TNF-R2 are “death receptor” (DR) pathways, but their cytotoxic effects are triggered via different intracellular mechanisms. Binding of the ligand TNF to TNF-R1 appears to cause very rapid depletion of intracellular anti-oxidants and death by oxidative stress. Binding of TNF to TNF-R2 causes downstream signaling that culminates in activation of the executioner caspases, caspase-3, caspase-7 and caspase-9, resulting in apoptosis. Click the diagram on the left for more detail on the TNF-R2 pathway.

In the cancer patient we see high concentrations of the soluble forms of TNF-R (sTNF-R1 and sTNF-R2) – plus a few other receptor types – in body fluids, apparently originating in the tumor micro-environment. sTNF-Rs are truncated versions of membrane TNF-Rs, consisting of only the extracellular binding domain (the ectodomain). Although sTNF-Rs necessarily lose their signaling capacity given that they are disengaged from the cell surface, they maintain full binding capacity. sTNF-Rs intercept the TNF/LT cytokines before they can bind with tumor cell surface receptors, thus neutralizing TNF/LT in the tumor micro-environment.

Clinical Approach

The basis of our clinical intervention is to employ apheresis coupled with an affinity column containing a proprietary biologic to remove inhibitory receptors from patient plasma in a controlled manner, thereby disrupting immunoevasion in the tumor microenvironment and inducing controlled tumor inflammation and necrosis. The treatment is purely subtractive and highly specific, resulting in the removal of target inhibitors and nothing else. The biologic we employ is a recombinant molecule with high binding affinity for target receptors but without cell signaling capacity, for safety reasons.

Clinically, we have observed that the rate of tumor destruction is a function of the level to which we reduce immune inhibitors in patient plasma and the duration for which we maintain reduced levels of these inhibitors. OncoPherese has the routine potential to destroy cancerous tissue faster than a patient can eliminate the resulting necrotic debris via the kidneys and other eliminative organs, creating a constant risk of Tumor Lysis Syndrome (TLS) – a condition rarely seen with other cancer treatments, which lack the efficacy of OncoPherese. We avoid TLS by constantly monitoring various markers of inflammation including C-reactive protein (CRP) – which rises in blood in response to inflammation – then reducing the rate of plasma flow through the affinity column as needed to attain a tumor destruction rate greater than that patient’s tumor growth rate but lower than the rate likely to lead to TLS. This can be clinically challenging, especially for patients with high tumor loads and rapid doubling rates.

For more information on the clinical aspects of OncoPherese, see our efficacy data, case reports, and “For Physicians” page.