CD244 and CD48

What is CD244?

• Member of signalling lymphocyte activation molecule (SLAM) family of receptors - particularly NK cells and cytotoxic T cells. 

• It is also found on myeloid cells such as: macrophages, dendritic cells, myeloid-derived suppressor cells, eosinophils, and basophils.

How does CD244 function?

• The same receptor can deliver activating or inhibiting signals, depending on molecular context.

• This augments cytotoxicity and cytokine production (lymphocyte activation).

• This dual function is central to nearly all later functional interpretations.

How does CD244 work?

• It has immunoreceptor tyrosine-based switch motifs (ITSMs) in its intracellular tail. Phosphorylation of these motifs upon binding its ligand will recruit intracellular adaptors.

• Recruitment of SLAM-associated protein (SAP) family adaptors such as SAP (itself) and EAT-2 tend to favour activating outcomes because this family couples activating kinases and prevents recruitment of inhibitory phosphatases.

• In the absence of SAP, inhibitory outcomes are favoured. Phosphatases like SHP-2 and SHIP bind to the ITSMs, supposedly triggering inhibitory signals - recruit inhibitory molecules and dampens signalling.

What molecular context affects CD244 function?

• SAP and EAT-2 expression differs depending on cell types and activation states. 

• CD244 isoforms.

• Ligand expression levels/ligand density.

• Whether CD244 interacts with its ligand in trans (between distinct cells) or cis (on the same cell surface) conformations. 

• Ligand-induced receptor modulation (downmodulation/internalisation).

• CD244 signalling is integrated with other receptor inputs (co-stimulation). For example, combined TCR and CD244 signals produce dynamic regulation - CD244 downmodulation on antiviral T cells during active signalling.

• Different disease-specific environments produce different cytokines, ligands etc.

How does Schlaphoff et al. (2011) demonstrate the dual role of CD244?

CD244 has a dual function in regulating HCV-specific CD8+ T cells: it can limit effector function (inhibitory outcome) or support activity depending on the cell state and co-receptors. This helps explain why T cell responses in chronic HCV are dysfunctional but not uniformly exhausted by a single mechanism.

How does Aldy et al. (2011) demonstrate the role of CD244 in HIV?

CD244+ CD8+ T cells can play an inhibitory role against constrained HIV epitopes, highlighting a setting where CD244+ expression marks cells with reduced capacity to clear certain viral variants.

How does Pacheco et al. (2013) demonstrate the role of CD244 in dynamic regulation during antiviral responses?

Simultaneous TCR and CD244 signals induce dynamic downmodulation of CD244 on human antiviral T cells - a mechanism that would transiently reduce CD244 signalling during strong antigen stimulation and thus change the functional outcome.

How was CD244 signalling implicated in immune cells within the TME by Agresta, Hoebe & Janssen (2008)?

• CD244 expression on tumour-infiltrating NK cells and T cells can influence anti-tumour activity.

• In some tumours or chronic contexts, CD244 signalling skews to inhibition (limiting cytotoxicity), contributing to immune evasion. In other contexts, engagement of CD244 may promote effector functions.

• The TME’s altered adaptor expression, persistent antigen, and suppressive cytokines can push the balance toward inhibitory signalling

How is ligand-induced modulation of receptor expression demonstrated?

• Sandusky et al. (2006) demonstrated that ligand binding itself regulates CD244-mediated NK cell activation via receptor modulation; ligand engagement can reduce surface CD244 expression, thus NK cell activation thresholds shift - the cell becomes less responsive to subsequent stimulation through CD244 (prevents overactivation).

• Mathew et al. (2009) also highlight the importance of isoforms: different human CD244 isoforms are generated via alternative splicing. These differ in the cytoplasmic tail composition. Some isoforms contain multiple ITSMs which recruit SAP, others have fewer or altered motifs which recruit inhibitory molecules. 

What is human CD244 isoform B?

• It is expressed in human NK cells, primary monocytes, basophils, and T cells.

• It contains additional amino acids between the Ig1 and Ig2 domains of CD244.

What is T cell exhaustion and how does CD244 contribute?

• It is a state of immune cell dysfunction where T cells become overworked from prolonged exposure to antigens in chronic infections or cancer, leading to a loss of their ability to fight off threats.

• In chronic viral infections (HCV, HIV) and in cancer, CD8+ T cells upregulate multiple inhibitory receptors; PD-1, TIM-3, LAG-3, and importantly CD244.

• CD244 contributes to exhaustion by altering the balance of activating vs inhibitory signalling through its cytoplasmic motifs.

What are the functional implications as a result of CD244-mediated T cell exhaustion?

• Reduced effector function - exhausted T cells fail to lyse target cells effectively and secrete fewer antiviral cytokines.

• Persistence of dysfunctional cells - instead of being deleted, exhausted T cells survive but remain hyporesponsive, maintaining high CD244 as a “signature.”

• Impaired memory formation - CD244hi exhausted T cells poorly transition into functional memory T cells, limiting durable immunity.

What are the roles of CD244 in other immune cells?

Current literature suggests that CD244 plays a multifaceted role in various immune cells, including macrophages and dendritic cells, during viral infections, with the potential to influence maturation, antigen presentation, and adaptive immune responses

What is CD48?

• It is a member of the SLAM family and is the primary ligand for CD244.

• It is a glycosylphosphatidylinositol (GPI)-anchored protein meaning it is tethered to the membrane by a lipid anchor, not a transmembrane domain. This limits its direct signalling capacity but allows it to cluster in lipid rafts and interact with signalling partners.

• It is widely expressed on haematopoietic cells with constitutive expression on most leukocytes, including T cells, B cells, NK cells, dendritic cells, macrophages, and granulocytes. It is upregulated during immune activation, infection, allergy, and malignancy. Expression differs in mice.

What are the therapeutic implications of CD244?

• Because CD244 can restrain effector function in chronic infection and cancer, blocking inhibitory CD244 pathways (or preventing inhibitory adaptor recruitment) is conceptually similar to immune checkpoint blockade. Turning an inhibitory CD244 signal into an activating one might restore NK/T cell activity in tumours or chronic infections.

• Conversely, in settings (e.g., boosting NK cell responses against tumours), approaches that favour SAP/EAT-2 recruitment or mimic activating adaptor function could amplify cytotoxicity.

• The context dependence suggests the need to design therapies that selectively modulate CD244 function only in specific cell types or tissue (molecular contexts).

What are the caveats of targeting CD244 therapeutically?

• Intervening in CD244 signalling risks unintended consequences due to its dual function.

• Most mechanistic work uses model systems; isoform differences and adaptor expression vary between humans and mice, so translational extrapolation must be cautious.

• Because CD244 is dynamically downmodulated by signalling, static measures of expression may poorly predict function - therapeutics must account for receptor plasticity.

What remains unclear about CD244?

• The regulatory mechanisms leading to CD244-mediated inhibitory signalling pathways are not well defined, particularly in myeloid cells that lack SAP expression.

• Similarly, the capacity of CD244 to internalize has been demonstrated in T and NK cells, but has not been previously studied in macrophages.

• Another key aspect not yet explored in the study of the CD244-CD48 axis in myeloid cells is the functional role of cis interactions.

• Little is known about the role of CD244 Isoform B, although it is thought to represent a negative feedback mechanism to attenuate the stimulatory signal from CD244-CD48 interaction and limit excessive NK cell activation.

• Although several studies have examined the functional role of CD244 in exhausted cells, it remains a controversial topic. In most of these studies, it is speculated that high CD244 expression combined with low SAP levels may significantly contribute to establishing T cell exhaustion.