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The mysterious function of CD4 protein: Why does it play a key role in the immune system?
In the immune system, CD4 protein plays a pivotal role, especially in regulating and coordinating immune responses. Since its discovery in the late 1970s, CD4 not only serves as a coreceptor for T cell receptors but is also involved in a variety of important biological processes. In this article, we will take an in-depth look at the structure, function, and role of CD4 in various diseases.
CD4 is a glycoprotein expressed on helper T cells, monocytes, macrophages and dendritic cells.
Structure of CD4
CD4 protein has a delicate and efficient structure and belongs to the immunoglobulin superfamily. Its structure consists of four immunoglobulin regions (D1 to D4), of which D1 and D3 are similar to the immunoglobulin variable domain (IgV), while D2 and D4 are similar to the immunoglobulin constant domain (IgC). The D1 region of CD4 interacts with the β2 region on MHC class II molecules, and this binding is the basis of the immune response.
The cytoplasmic tail of CD4 contains a specific amino acid sequence that enables it to recruit and interact with the tyrosine kinase Lck.
Function of CD4
CD4 mainly serves as a coreceptor for T cell receptors (TCR), promoting communication between TCR and antigen-presenting cells. The tight binding between CD4 and the TCR complex can induce the activation of tyrosine kinase Lck, thereby amplifying the signal generated by the TCR. In addition, CD4 activation also guides downstream signaling chains, including activation of transcription factors, which is critical for T cell activation.
These signals will ultimately promote the comprehensive activation of T cells and are the core of the entire immune response process.
The relationship between CD4 and disease
HIV infection
HIV-1 uses CD4 to enter host T cells, a process that relies on its viral envelope protein gp120. When gp120 binds to CD4, it causes its conformational changes and promotes the binding of HIV-1 to the host cell co-receptor. The end result of this process is the fusion of the viral membrane with the host cell membrane, resulting in infection.
As HIV infection progresses, the number of CD4-positive T cells gradually decreases, causing the patient's immune system to be severely compromised.
Other diseases
In addition to HIV, CD4 plays a key role in many diseases, such as autoimmune diseases and tumor progression. CD4-positive T cells are consistently expressed in various cancers, making them one of the markers for peripheral T-cell lymphoma in tissue biopsy samples. In addition, studies have shown that CD4 is associated with a variety of autoimmune diseases, which further highlights the importance of CD4 in disease diagnosis and treatment.
For example, when testing the effects of drugs or studying disease, it is particularly important to quantify the numbers of CD4+, CD8+, and CD3+ T cells.
Summary
The role of CD4 protein in the immune system is undoubtedly multidimensional and complex. From immune regulation to pathological effects, CD4 is not only a key driver of immune responses but also an important marker in the study of many diseases. As scientists further explore the functions of CD4, we may be able to unlock more biological mysteries related to it. In the future, will CD4 become the key to overcoming various diseases?
