Beyond T Cells: Harnessing Innate Immunity to Prevent Transplant Rejection

When a transplanted organ arrives, it’s like a controlled burn that risks becoming a wildfire. The body’s innate immune system senses damage signals, like heat shock proteins (HSP70), and sounds the alarm, mobilizing dendritic cells to fan the flames of inflammation. Antigen-presenting cells rally T cells to the scene, launching an attack on the foreign tissue. These are the first steps of organ transplant rejection.

Current immunosuppressants act like firefighters putting out the flames once the building (in our case, the tissue) is a blaze. In a new study, Siglec-E in mice (and Siglec-7/9 in humans) acts as an early intervention alert, preventing those fires from spreading in the first place. This receptor binds sialic acid ligands to keep dendritic cells from overreacting. By blocking NF-κB signaling and dampening pro-inflammatory cytokines like TNF-α, Siglec-E keeps the immune response from spiraling out of control.

Without these inhibitory receptors, the immune response surges unchecked, leading to heightened inflammation, accelerated T-cell activation, and faster transplant rejection. By targeting this upstream checkpoint, the researchers propose, it may be possible to quiet the inflammatory blaze at its source—offering a possible therapeutic strategy to protect transplanted organs without broadly suppressing immune function.

Innate immunity’s role in rejection

Current immunosuppressive therapies primarily target T cells, the drivers of the adaptive immune response that recognizes and attacks transplanted organs. These treatments, while effective at reducing rejection, come with a high cost: they broadly suppress immunity, leaving patients vulnerable to infections, cancers, and other complications. Yet despite this aggressive suppression of T cells, many transplants still fail over time. Increasing evidence suggests that early inflammation, mediated by the innate immune system, plays a pivotal role in setting the stage for rejection.

Recognizing this gap, the Mass General Brigham researchers turned their attention upstream, to the body’s first line of defense. Rather than focusing solely on dampening T cells, they explored whether controlling the innate immune response could prevent the inflammatory cascade from spiraling in the first place. By investigating the inhibitory receptor Siglec-E in mice—and its human counterparts, Siglec-7 and Siglec-9—they identified a natural checkpoint that calms overactive immune responses early on.

To test the role of Siglec-E, the team used preclinical mouse models of heart, kidney, and skin transplantation. They found that mice lacking Siglec-E experienced accelerated rejection, heightened inflammation, and increased activation of dendritic cells—the antigen-presenting cells that bridge innate and adaptive immunity. Without Siglec-E, dendritic cells stayed hyperactivated, producing more pro-inflammatory cytokines like TNF-α and IL-6, and driving stronger T-cell responses against the allograft.

When the researchers analyzed human transplant samples, they observed that higher levels of Siglec-7 and Siglec-9 were associated with better graft survival. This finding suggests that the protective role of these inhibitory receptors extends to humans, offering translational potential for new therapies.

By identifying this natural “brake” in the immune system, the study points to a new therapeutic strategy: targeting Siglec-7 and Siglec-9 to modulate dendritic cell activation, reducing inflammation without globally shutting down the immune system. Instead of waiting to suppress T cells after the immune system is already fully mobilized, therapies directed at this pathway could quiet the alarm at its source, preventing the inflammatory cascade that leads to rejection.

A promising target for next-generation transplant therapies

“Regulating innate immune activation is a crucial step in the prevention of transplant rejection and improvement in transplant outcomes,” the researchers state. Prolonged innate immune responses may reduce tolerance while interfering with trained immunity and the adaptive immune response. To maintain a balanced immune response, the innate immune response must be recognized as a front-line defender against excessive inflammation and possible tissue damage.

“By harnessing natural inhibitory pathways like Siglec-E, we can develop safer, more precise therapies that protect transplanted organs without compromising overall immune health,” said Leonardo Riella, MD, PhD, medical director of kidney transplantation at Massachusetts General Hospital.

As a negative regulator of innate immune responses and acute T cell-mediated transplant rejection in mice, Siglec-E offers a potential therapeutic target that may be translatable to humans. This offers hope for a next generation of organ transplant treatments, including longer-lasting transplant success and reducing the need for lifelong immunosuppression.