Cancer Immunotherapy II
Cancer vaccines represent a highly active and developing area within cancer therapeutics, with specific prominence at institutions like the University of Florida (UF), which maintains a strong immunotherapy research group. This means that scientists are working hard to create vaccines that can help the body fight cancer, similar to how vaccines prevent diseases like the flu.
The fundamental mechanism of dendritic cell vaccines involves priming and activating dendritic cells with tumor-specific antigens to enhance the body’s T cell immunity. To understand this, it's helpful to know what dendritic cells and antigens are. Dendritic cells are a type of immune cell that helps alert the body about threats, such as cancer or infections. Tumor-specific antigens are unique markers found on cancer cells that can trigger an immune response.
The Process of Dendritic Cell Activation:
Dendritic cells are isolated from the patient. This step involves taking a sample of the patient’s own blood to find these specialized cells.
These cells are ‘pulsed’ with tumor antigens, a process that triggers their activation and maturation. This means that the researchers introduce those unique cancer markers to the dendritic cells, essentially training them to recognize the cancer.
Once activated and matured, the dendritic cells are reinjected into the patient. This is similar to how vaccines work; the patient receives a dose of activated cells that can better identify and destroy cancer cells.
Therapeutic Action:
The reinjected dendritic cells prime and activate cytotoxic T cells. These T cells are a crucial part of the immune system that directly kill infected or cancerous cells. After training, these T cells patrol the entire body to identify and eliminate tumors that express the specific antigens they were primed against.
Advantages of the ‘Pulsing’ Approach:
It is an ‘agnostic’ use, meaning clinicians do not necessarily need to identify the specific tumor antigen beforehand. This is important because it can be complicated to find the exact tumor markers in every patient’s cancer.
There is no requirement to separate self-antigens from neoantigens. Self-antigens are those that are recognized by the immune system as normal body cells, while neoantigens are unique to tumor cells due to mutations.
The immune system naturally tolerates self-antigens, ensuring that only the tumor-specific antigens induce and potentiate the anti-tumor response.
Sipuleucel-T and Recombinant Fusion Proteins
Sipuleucel-T is a prominent example of an FDA-approved dendritic cell vaccine, used specifically for metastatic castration-resistant prostate cancer. This means it’s a treatment for a specific type of prostate cancer that is advanced and does not respond to hormonal therapy anymore.
In principle, this therapy is a form of cell therapy, which represents a newer way of treating diseases that focuses on using a person’s own cells.
Production and Mechanism:
Peripheral blood mononuclear cells (PBMCs) are isolated from the patient. This means scientists take some of the patient’s blood to extract certain immune cells.
From these PBMCs, monocytes are selected out. Monocytes are a type of white blood cell that can differentiate into other types of immune cells.
The cells are activated using a recombinant fusion protein consisting of Prostatic Acid Phosphatase (PAP) fused to Granulocyte-Monocyte Colony-Stimulating Factor (). PAP is an antigen expressed in many prostate cancers, and is a signaling molecule that helps the immune cells mature.
The cells are cultured, pulsed with the fusion protein, and re-infused into the patient. This means the cells are grown in the lab until they’re ready to be used again in the patient.
Outcome:
The infused monocytes, having been exposed to the antigen and signaling factor, differentiate into activated APCs. These are Antigen-Presenting Cells that play a vital role in alerting T cells about the presence of cancer. These cells present the antigen to cytotoxic T cells, which are then primed for direct tumor lysis. Tumor lysis is the process of breaking down and destroying the cancer cells.
Engineered Oncolytic Viruses
Oncolytic viruses represent an expanding area of research, including significant work being performed at UF. Research in this area is exciting because scientists are finding ways to use viruses to treat cancer.
Currently, one such virus is FDA-approved (T-VEC) for the treatment of recurrent unresectable melanoma. Melanoma is a serious form of skin cancer.
Unlike many systemic immunotherapies, these must be injected directly into the tumor. This means that, rather than taking pills or getting infusions, the treatment is delivered exactly where it’s needed.
The Mechanism of Tumor Specificity:
Normal cells have evolved complex anti-viral mechanisms, primarily centered on interferon-based pathways, to resist infection and shut down viral mRNA/protein expression. This is like having shields to protect the body from harmful viruses.
Tumor cells often lose these anti-viral responses as a ‘bystander effect’ of tumorigenesis. Because tumors prioritize unlimited growth, they may shut down non-essential antiviral pathways that might otherwise interfere with active proliferation. This means that cancer cells can sometimes be more vulnerable to viruses that normal cells can fight off.
Without these protections, oncolytic viruses can easily infect, replicate within, and lyse (burst) tumor cells, whereas normal cells remain immune to viral lysis.
T-VEC (Talimogene Laherparepvec):
It is an engineered virus that replicates specifically in tumor cells, meaning it’s been modified to target and attack cancerous tissue.
It is engineered to carry the gene for .
The ‘Chain Reaction’ of T-VEC:
The virus replicates in and lyses the cancer cell.
Lysis releases a high volume of tumor-specific antigens and the engineered .
Dendritic cells take up the antigens and are further activated by the .
These dendritic cells educate T cells to recognize and destroy cancer cells throughout the body (abscopal effect), not just at the injection site. This means that the immune response can target cancer cells in other parts of the body as well.
Immune Checkpoint Inhibitors
The development of immune checkpoint inhibitors was recognized with the Nobel Prize awarded to Tasuku Honjo and James Allison for their descriptions of the mechanisms governing immune checkpoints. These inhibitors are a powerful tool for treating cancer by helping the immune system recognize and attack cancer cells.
The Priming Phase (Lymph Node):
Antigen presentation involves the Major Histocompatibility Complex (MHC) Class I presenting antigens to T cell receptors (TCR). This interaction is crucial for activating T cells against cancer.
Activation requires a secondary signal: the binding of on the T cell to (specifically or ) on the dendritic cell. This two-step activation process is essential for a robust immune response.
The CTLA-4 Checkpoint: Shortly after activation, T cells express , an inhibitory molecule. also binds to , but provides an inhibitory signal to prevent over-activation.
High tumor antigen loads often lead to chronic overstimulation and subsequent immune inhibition.
Anti- antibodies (e.g., Ipilimumab) block the inhibitory interaction, allowing the co-stimulatory signal to persist and activate T cells. This blockade is key because it enhances the T cells' ability to attack the tumor.
The Effector Phase (Peripheral Tissue):
In peripheral tissues, the TCR recognizes tumor antigens on the tumor cell surface via MHC Class I. This means that T cells can spot and identify cancer cells as harmful.
The PD-1/PD-L1 Checkpoint: T cells express the receptor. Tumor cells, under immune pressure, often evolve to overexpress the ligand (Programmed Death Ligand 1).
The and interaction inhibits the T cell’s cytotoxic function. This is a way that tumors try to escape the immune response.
Anti- or anti- antibodies (e.g., Pembrolizumab, Atezolizumab) break this interaction, allowing T cells to carry out tumorlytic functions.
Key Distinction:
inhibition happens in the lymph node during the priming phase between dendritic cells and T cells.
/ inhibition happens in the peripheral tissue between the T cell and the target (cancer) cell. Understanding these differences is important for targeting treatments effectively.
Mutational Load and Clinical Response
For the immune system to target a tumor, the tumor must have neoantigens distinct from self-antigens. This distinction is critical because the immune system can more readily attack cells that present unfamiliar markers.
Mutational Prevalence: The extent of mutagenesis varies widely across tumor types, sometimes by more than . Some cancers mutate more than others, which can influence treatment effectiveness.
Correlation: Generally, a higher mutational load per megabase in a tumor correlates with a higher objective response rate to checkpoint inhibitors. Highly mutagenic tumors produce more neoantigens for the immune system to recognize, thereby increasing the likelihood of a therapeutic effect.
Mismatch Repair (MMR): - MMR-deficient tumors rapidly accumulate point mutations and short repeat expansions and are highly responsive to checkpoint inhibitors. This means that some cancers that can’t fix their DNA mistakes are more easily attacked by immunotherapies.
MMR-proficient tumors (like some colorectal cancers) have low mutation counts and show dismal responses.
The Merkel Cell Carcinoma Outlier: This skin cancer has a relatively low mutational load in its host genome but a high response rate to checkpoint inhibitors. This is because it is predominantly driven by the polyomavirus; the viral antigens themselves drive the immune response. Understanding unique tumor responses can help tailor specific treatments for individual patients.
Chimeric Antigen Receptor (CAR) T Cell Therapy
CAR T cells represent a fusion or ‘chimaera’ of a target-binding domain and internal co-stimulatory domains. This design enables the T cells to identify and kill cancer cells more effectively by streamlining how they recognize their targets.
The Challenges of CAR T:
Antigen Specificity: Requires a known, ideally universal, tumor antigen. It’s essential that the therapy targets markers that are widely present on cancer cells.
Solid Tumors: CAR T therapy is currently less effective in solid tumors due to the highly immunosuppressive microenvironment. This means that the environment around solid tumors can hinder T cell activity. Research is ongoing to engineer CAR T cells that express cytokines to combat this.
Cell Lineage Toxicity: Current success is limited to B cell malignancies. Myeloid malignancies are harder because humans cannot survive without myeloid cells.
Safety Mechanisms: Due to concerns about killing healthy cells (e.g., healthy myeloid cells), researchers are investigating ‘suicide switches’ to turn off CAR T cells after they fulfill their function. This helps to minimize any unintended damage to healthy tissues.
Manufacturing (Autologous Gene Therapy):
Patient T cells are isolated and sent to a manufacturing facility. This is where the specialized treatment is created.
A retrovirus is used to deliver the CAR gene into the T cells. This is a method of changing the T cells' genetic makeup to help them target cancer better.
The modified cells are expanded over approximately and then shipped back to the hospital for infusion, which allows enough time to create a sufficient number of treated cells for therapy.
Current FDA Approvals (2017): Anti- CAR T cells are approved for pediatric and adult relapsed/refractory B cell malignancies. This is a breakthrough in cancer treatment as it provides new hope for patients with challenging types of leukemia.
Management of CAR T Toxicities
Toxicities associated with CAR T cells are prevalent, occurring in more than half of patients. It’s essential to monitor patients closely during and after treatment.
Cytokine Release Syndrome (CRS): - A result of over-activation of the immune system.
Symptoms: Dangerously high fever, precipitous drop in blood pressure, organ toxicity, and neurotoxicity. These symptoms can be quite serious and require immediate medical attention.
Management: Tocilizumab (an anti- antibody) is the primary treatment, as Interleukin-6 is the main driver of CRS. Steroids are added for severe neurotoxicity or acute organ injury.
On-Target/Off-Tumor Toxicities:
B Cell Aplasia: Depletion of healthy B cells leads to an immunocompromised state, meaning the patient's ability to fight infections decreases.
Viral Reactivation: Risk of reactivating endogenous viruses like Hepatitis B.
Management: Managed via prophylactic antibiotics and immunoglobulin infusions to replace lost antibodies, ensuring patients maintain some immune protection while undergoing therapy.
The Tumor Immunity Cycle Summary
Step 1: Release of Antigens. Caused by tumor death from chemotherapy, radiation, or other therapies. This is the first step in awakening the immune system.
Step 2: Antigen Presentation. Antigens are processed by dendritic cells. (Target for vaccines, Interferon , and ).Dendritic cells play a crucial role in alerting T cells to the presence of cancer cells.
Step 3: Priming and Activation. Occurs in the lymph node. (Target for and Anti-). This is when T cells get activated and prepared to search for cancer cells.
Step 4 & 5: Trafficking and Infiltration. T cells travel to the tumor site; they get to where the cancer is present.
Step 6: Recognition. T cells identify cancer cells. (Target for CAR T engineering). This is the critical moment when T cells find their targets.
Step 7: Killing. T cells lyse the tumor. (Target for Anti- and Anti- checkpoint inhibitors). This final step is when T cells effectively destroy cancer cells, leading to either tumor regression or resolution.