How a shuttling protein directs T cell development and immune function
Imagine your body as a bustling city constantly defending against invisible invaders. The T cells, the elite security forces of this city, rely on precise instructions to identify friends from foes. For years, scientists knew these instructions existed but couldn't identify all the messengers. Then they discovered Themis—a mysterious protein that acts as a crucial director in the immune system's command center.
What makes Themis particularly fascinating is its constant shuttling from the cytoplasm to the nucleus, a dynamic dance that proves essential for proper immune function. Recent breakthroughs have finally begun to reveal how this molecular commute allows Themis to conduct the complex symphony of your immune defense.
Discovered simultaneously by several research groups in 2009, Themis (short for "thymocyte-expressed molecule involved in selection") quickly established itself as a non-negotiable component in T cell development. Its name, derived from Themis, the Greek goddess of divine law and order, reflects its regulatory role in maintaining order in the immune system.
Themis belongs to an exclusive family of proteins found only in T cells, the white blood cells that coordinate multiple aspects of immune defense and directly attack infected cells.
What makes Themis structurally intriguing are its distinctive components:
These structural elements work together to enable Themis to respond to cellular signals and execute its functions 7 .
Early observations revealed something puzzling about Themis: it doesn't stay in one place within the T cell. Instead, it constantly shuttles between the cytoplasm (the gel-like substance filling the cell) and the nucleus (the command center containing DNA). This wasn't random wandering—it appeared crucial for Themis to perform its job, though initially, scientists didn't understand why.
The nuclear localization signal acts as Themis's passport for nuclear entry. Think of it as a security badge that grants access to secure facilities within the cell. Without this badge, Themis remains stuck in the cytoplasm, unable to influence the genetic programming that occurs in the nucleus 7 .
This shuttling behavior represents one of immunology's intriguing puzzles: how does moving between cellular compartments help Themis regulate T cell function? The answer would require clever experiments to unravel.
Themis (blue dot) shuttles between cytoplasm and nucleus in T cells
To decode Themis's secrets, Dr. Kiyokazu Kakugawa and colleagues employed a systematic approach to investigate how each part of Themis contributes to its function. Their elegant experiments involved creating mutant versions of Themis and observing what happened when these mutants were introduced into cells lacking normal Themis 7 .
The research team followed these key steps:
The experiments yielded clear and compelling results, summarized in the table below:
| Mutant Type | Nuclear Localization | Grb2 Binding | TCR Signaling | Rescue of T Cell Development |
|---|---|---|---|---|
| Wildtype Themis | Normal (+++) | Normal (+++) | Normal (+++) | Full rescue |
| ΔNLS | Severely impaired (+) | Absent (-) | Impaired (+) | No rescue |
| ΔCore1 | Severely impaired (+) | Absent (-) | Impaired (+) | No rescue |
| ΔCore2 | Normal (+++) | Absent (-) | Impaired (+) | No rescue |
| ΔPRS | Normal (+++) | Absent (-) | Impaired (+) | No rescue |
| CAB2-1 | Normal (+++) | Normal (+++) | Normal (+++) | Partial rescue |
The data revealed a striking pattern: both nuclear localization and interaction capabilities proved essential for Themis function. Themis mutants that couldn't reach the nucleus (ΔNLS and ΔCore1) failed to support T cell development, demonstrating that nuclear access is non-negotiable. Similarly, mutants that couldn't interact with key signaling proteins like Grb2 (all deletion mutants) also failed, regardless of their location 7 .
Perhaps most intriguingly, the Core1 and NLS domains served dual purposes—both were essential for nuclear localization, suggesting they work together as part of Themis's nuclear import machinery.
| Experimental Condition | CD4+ T Cells Produced | CD8+ T Cells Produced | Overall T Cell Development |
|---|---|---|---|
| Normal Mice | 100% | 100% | Normal |
| Themis-Deficient Mice | 20-30% | 20-30% | Severely impaired |
| + Wildtype Themis | 85-95% | 85-95% | Mostly restored |
| + ΔNLS Mutant | 20-30% | 20-30% | No improvement |
| + ΔCore1 Mutant | 20-30% | 20-30% | No improvement |
| + ΔCore2 Mutant | 20-30% | 20-30% | No improvement |
Comparison of T cell development rescue efficiency across different Themis mutants
The implications were profound: Themis isn't a one-location protein. It must physically commute between cellular compartments to coordinate the complex signals that determine T cell fate. This shuttling allows it to gather information at the cell surface (where T cell receptors interact with antigens) and relay instructions to the nucleus (where genetic programs are executed).
Studying a complex protein like Themis requires specialized tools. The table below highlights key reagents that enabled researchers to decode Themis's functions:
| Research Tool | Function in Themis Research | Key Discoveries Enabled |
|---|---|---|
| Themisflox/flox dLck-Cre mice | Enables selective deletion of Themis in T cells after thymic development | Revealed Themis roles in mature T cells, separate from developmental effects |
| LCMV Clone13 model | Chronic viral infection model that persists in hosts | Demonstrated Themis's time-dependent regulation of T follicular helper cells during chronic infection 1 6 |
| Single-cell RNA sequencing | Analyzes gene expression in individual cells | Identified Themis's role in CD4+ T cell progenitor differentiation 1 |
| Grb2-binding assays | Measures interaction between Themis and signaling protein Grb2 | Confirmed Themis integrates TCR signaling via Grb2 interaction 7 9 |
| Nuclear-cytoplasmic fractionation | Separates cellular components to determine protein localization | Established Themis's dynamic shuttling between compartments 7 |
Conditional knockout mice allow researchers to delete Themis at specific developmental stages or in specific cell types, revealing its stage-specific functions.
Protein interaction studies and localization techniques help map Themis's molecular partnerships and cellular journey.
Themis's influence extends far beyond initial T cell development. Recent research has revealed that Themis plays a dual-stage regulatory role during chronic viral infections. During early infection stages, Themis promotes the formation of T follicular helper (Tfh) cells, which are essential for antibody production. Surprisingly, during late infection stages, Themis does an about-face and actually restrains excessive Tfh differentiation 1 6 .
Themis shows opposite effects on Tfh cell differentiation during early vs. late chronic infection
This temporal switching suggests Themis functions as a sophisticated immune rheostat, dynamically adjusting T cell responses throughout an infection. Without this regulation, the immune system might overrespond or underrespond, either causing collateral damage or allowing pathogens to persist.
Themis also plays a critical role in T cell exhaustion—a dysfunctional state that occurs during chronic infections and cancer. When Themis is dysfunctional in CD8+ T cells, it disrupts the normal exhaustion process, leading to 80% mortality in mouse models. This highlights Themis's potential as a therapeutic target for chronic infections and cancer immunotherapy 1 3 .
Themis regulates immune response dynamics
Potential target for immunotherapy
Could help modulate excessive immune responses
Themis represents a fascinating example of how molecular commuting within cells—dynamic translocation between compartments—can govern life-or-death decisions in the immune system. Its shuttling between cytoplasm and nucleus allows it to integrate signals from the environment and translate them into genetic programs that determine T cell fate.
The meticulous work of Dr. Kakugawa and other researchers in mapping Themis's functional domains has opened new avenues for understanding immune regulation. Their findings not only solve a basic science mystery but also suggest future medical applications. By understanding how Themis works, we might eventually learn to manipulate its activity—potentially boosting immunity against infections and cancer or calming excessive immune responses in autoimmune diseases.
As research continues, each discovery about Themis's intricate functions reminds us of the astonishing complexity within every T cell in our bodies, and how much remains to be learned about the molecular conductors that orchestrate our immune defense.