Fox Chase Researchers Characterize Structure of Receptor That Helps Mediate Activity of Natural Killer Immune Cells

PHILADELPHIA (August 16, 2019) – Natural killer cells, which have an innate capacity to identify and kill tumor cells or virus-infected cells in humans, are an important part of the body’s immune system. Key in distinguishing healthy cells from abnormal cells by natural killer cells is their expression of killer cell immunoglobulin-like receptors (KIRs).

Researchers at Fox Chase Cancer Center recently published a paper detailing the use of a powerful biophysics technique, nuclear magnetic resonance (NMR), to gain a better understanding of the structure of one KIR – KIR3DL1 – and to characterize its conformational dynamics.

There are 15 KIR family member proteins said Kerry S. Campbell, PhD, professor in the Blood Cell Development and Function Program at Fox Chase. Some of the proteins have an activating function and others have an inhibitory function.

When they encounter normal cells, inhibitory KIRs transmit a signal that shuts down the ability of the natural killer cell to attack. This signal comes from KIR-detecting major histocompatibility complex class I (MHC-I) molecules, which are found on the surface of virtually every healthy cell of the body but missing on many tumor and virus-infected cells.

“MHC-I acts as a safety beacon that these KIRs recognize,” Campbell said. “KIRs on natural killer cells encounter a lot of cells in the bloodstream and tissues, and sample them as they go by. These KIRs will recognize cells that have MHC class I on the surface, and when engagement occurs, it sends a signal to the natural killer cell to prevent them from attacking that cell.”

Specifically, when engagement occurs between KIRs and MHC-I, tyrosine residues in two immunoreceptor tyrosine-based inhibitory motifs (ITIMs) on the intracellular domain of KIR are phosphorylated by protein tyrosine kinases.

This process creates docking sites for the protein tyrosine kinase phosphatases SHP-1 and SHP-2 that contain a pair of phosphor-tyrosine recognition (SH2) domains. Recruitment of SHP-1 and SHP-2 transmits the inhibitory signal that tolerizes natural killer cells to the MHC-I expressing normal cells in the body. 

“Previously we knew that SHP-1 and SHP-2 bind to two tyrosines inside the cell, but we did not know their orientation and whether there was some existing structure in the cytoplasmic domain of KIR or if it was disordered,” Campbell said.

To explore this further, Campbell collaborated with Hong Cheng, PhD, and Heinrich Roder, PhD, biophysicists at Fox Chase, to employ a 13C-detected (NMR) technique ideally suited for characterizing dynamic macromolecules. Using this technique, they found that the intracellular region of KIR3DL1 is highly flexible and lacks specific conformation/structure. It comprises three regions with distinct secondary structure propensities, including a-helical (I), extended (II), and disordered (III) segments.

When exposed to hydrophobic conditions, the dynamic a-helical structure was observed not only in segment I, but also in segment III, particularly around one of the two ITIM tyrosines. This suggests that their reactivity with respect to kinases and phosphatases may be regulated via differential interactions of the intracellular tail of KIR3DL1 with the plasma membrane.

 Furthermore, the group used the 13C-NMR technique to map interactions of SHP-2 with the tyrosines under unphosphorylated and phosphorylated conditions by detecting acquisition of structure in these regions.

According to Campbell, understanding the mechanisms of how these receptors function offers opportunities to design small molecule therapies that could block the ability of KIRs to recruit phosphatases SHP-1 and SHP-2. Blockade of phosphatase recruitment could reduce KIR function and increase the activation potential of natural killer cells to attack tumors. 

There are a number of other receptors that have similar tyrosine motifs found in lymphocytes and other cells that send inhibitory signals. One of these is PD-1, a very common checkpoint receptor targeted in cancer immunotherapy.

There are ongoing clinical trials targeting KIRs with antibodies to block their function. KIRs are also considered a checkpoint receptor, but on natural killer cells instead of T cells.

“It is the same concept as turning on a T cell with anti-PD-1 drugs,” Campbell said. “Here you could turn on natural killer cells with anti-KIRs.”

The work was supported by National Institutes of Health grants CA083859 and GM116911, a Cancer Center Support Grant from the National Cancer Institute (CA06927), an appropriation from the Commonwealth of Pennsylvania, and the Spectroscopy Support Facility of Fox Chase Cancer Center

The paper, “Conformational Changes in the Cytoplasmic Region of KIR3DL1 Upon Interaction With SHP-2,” was published in Structure.

Fox Chase Cancer Center (Fox Chase), which includes the Institute for Cancer Research and the American Oncologic Hospital and is a part of Temple Health, is one of the leading comprehensive cancer centers in the United States. Founded in 1904 in Philadelphia as one of the nation’s first cancer hospitals, Fox Chase was also among the first institutions to be designated a National Cancer Institute Comprehensive Cancer Center in 1974. Fox Chase is also one of just 10 members of the Alliance of Dedicated Cancer Centers. Fox Chase researchers have won the highest awards in their fields, including two Nobel Prizes. Fox Chase physicians are also routinely recognized in national rankings, and the Center’s nursing program has received the Magnet recognition for excellence five consecutive times. Today, Fox Chase conducts a broad array of nationally competitive basic, translational, and clinical research, with special programs in cancer prevention, detection, survivorship, and community outreach. It is the policy of Fox Chase Cancer Center that there shall be no exclusion from, or participation in, and no one denied the benefits of, the delivery of quality medical care on the basis of race, ethnicity, religion, sexual orientation, gender, gender identity/expression, disability, age, ancestry, color, national origin, physical ability, level of education, or source of payment.

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