A new class of cancer treatments that harness the body's immune system to fight tumors is being hailed as the biggest thing in oncology since CAR-T revealed the promise of cell therapy more than a decade ago.

• But with price tags of hundreds of thousands of dollars, the drugs raise familiar concerns about affordability and access.

Why it matters: While CAR-T has been successful in some blood cancers, it's not yet been approved for solid tumors, which make up about 90% of cancer types. The new therapy class — tumor-infiltrating lymphocytes, or TIL — uses immune cells from a patient's tumor to mount a long-lasting defense.

Driving the news: The Food and Drug Administration last week gave the first-ever accelerated approval of a TIL to Iovance Biotherapeutics' melanoma therapy Amtagvi. It was hailed as a major milestone as a growing pipeline of TIL candidates could eventually be deployed against a range of other cancers.

• "This is the tip of the iceberg of what TIL can bring to the future of medicine," Patrick Hwu, CEO of Moffitt Cancer Center, told Axios.

How it works: TILs have been around for decades — National Institutes of Health researcher Steven Rosenberg is widely credited with discovering TILs in 1988 — but their use has been limited to several academic cancer centers.

• TIL therapy works by extracting patients' T-cells directly from tumors and essentially "giving them the Club Med treatment," said Jason Bock, CEO of CTMC, a joint venture between MD Anderson Cancer Center and biomanufacturing company Resilience.
• Patients' immune systems are temporarily weakened before providers infuse the strengthened and multiplied cells back into the patient.
• In a Phase 2 clinical trial of Amtagvi, 31.5% of patients responded to the treatment.
• In more recent NIH research on TIL therapy, Rosenberg told Axios there was a 56% response rate among patients with melanoma, and 24% of patients had a complete disappearance of their melanoma, regardless of where it was.
• Companies with TILs in early and mid-stage clinical trials include Obsidian Therapeutics, Instil Bio, Turnstone Biologics and Achilles Therapeutics. Their candidates are aimed at melanoma, as well as lung, ovarian and kidney cancers.

Iovance's new therapy, like other cell-based treatments, likely won't work for all patients. The drug's prescribing information warns of low blood cell counts and increased risks for infection.

• One of the greatest risks — the temporary lowering of the immune system — is common with cell therapies, said Brian Gastman, Iovance's executive vice president of medical affairs.
• Patients have to undergo chemotherapy and receive a protein known as interleukin-2, which primes the body for the TIL therapy and carries potential side effects including cardiopulmonary and renal function risks.

What to watch: The accelerated approval is expected to make the therapy more widely available.

• The FDA approved WuXi Advanced Therapies to begin manufacturing Amtagvi in its Philadelphia plant.
• Iovance expects at least 30 authorized treatment centers to begin administering Amtagvi almost immediately, with plans to grow that to 50 centers in three months.
• Part of what's taken so long to get to this point has been a lack of a clear regulatory pathway for scaling what is essentially a bespoke treatment.
• "We create a new drug for every patient," said Rosenberg, who's still at NIH.

Zoom in: Iovance has priced the treatment at $515,000, which makes it the most expensive of any cell-based therapy in the United States.

• The company looked to CAR-T treatments, which are typically priced in the hundreds of thousands of dollars, as a benchmark for how to price Amtagvi, Iovance's Gastman said.
• The therapy requires a resource-intensive 22-day process of collecting, cultivating and re-administering cells, Gastman said.
• He also noted that as a one-time treatment, it has an advantage over other drugs for melanoma that patients have to be on longer term.

What they're saying: "I am gratified to see the FDA finally recognize the value of it," Rosenberg said.

• "We're working around the clock in the lab now to try to extend these findings from melanoma to other tumor types and you're gonna see some important advances coming along the next couple of years," he said.