Introduction

COVID-19 is a coronavirus disease that emerged in 2019. This is caused by the RNA virus named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). It mainly affects the respiratory system of the human body and causes symptoms of respiratory illness like coughing, pyrexia, headache, fatigue and allergic signs like flu, rash, etc.

The main pathogenesis behind this COVID-19 is the activation and overactivation of the immune system after the viral infection by SARS-CoV-2.¹

How does lymphocytosis occur in COVID-19?

Upon entry into the human body, it stimulates immune cells and causes an inflammation. Among all the immune cells, lymphocytes are crucial in adaptive immunity. Initially, activation of lymphocytes–T cells and B cells, occurs and leads to the recruitment of more such cells to the desired infected site. This mechanism of overactivation leads to lymphocytosis and results in the killing of foreign viral antigens.^1,2

How is lymphopenia caused by SARS-CoV-2?

Persistent presence of SARS-CoV-2 or viral exposure leads to chronic inflammation.  Resultantly, overkilling of nearby healthy immune cells is being recruited there for the defence process.^1,2 This will ultimately kill lymphocytes, resulting in a lower count of lymphocytes than the normal range (less than 1,500 cells/μL of blood) in blood, and is called lymphocytopenia. This is also termed as lymphopenia.^1,2

How are low and high lymphocyte counts related to each other

In acute COVID-19, lymphocytosis occurs as part of the body's defence mechanism.² This is followed by lymphocytopenia in the chronic stage of the disease.² This is how both processes- first, an increase, then a decrease in adaptive immune cells– are interlinked.²

Pathophysiology of COVID-19?

When the SARS-CoV invades your body, it is detected as a foreign particle, pathogen-associated molecular patterns (PAMPs). This process of microbial invasion is called infection, which initiates immune-mediated signal transduction pathways leading to inflammation.¹

PAMPs act as a signal, the ligand. These need gate-pattern recognition receptors (PRRs) for the quick entry into immune cells of the human body. After the PAMPs-PRRs complex, an inflammatory process ensues.¹ 

COVID-19 case

SARS-CoV invades the cells in the following ways:

Angiotensin-converting enzyme type 2 (ACE2) receptor & Transmembrane protease serine-2 (TMPRSS2)

It is present on different types of human cells:^1,2,4,5

• Endothelial cells of the gastrointestinal tract (GIT)
• Astrocytes
• Ear cells – olfactory region
• Muscle cells
• Lungs- bronchiolar cells
• Skin 
• Eyes

Cytokines pool

Uncontrolled release of cytokines leads to more recruitment of interleukins – IL-17, IL-1𝝱, IL-2, IL-13, and IL-5.^1,4

Direct contact with toll-like receptors- TLR 3 or 4

TLRs act as PRRs (T-cell receptors- TCR), which are present on T-lymphocytes, leading to the release of the same cytokines.¹

Lymphocyte function-associated antigens (LFA-1)

These act as integrins, which activate T cells, leading to cytokine outbursts.²

Direct contact with NOD-like receptors (NLR)

Virtual components– NLR complexes translate mRNA of ILs and interferons (IFN) into respective proteins.³

Direct entry to T-cells

Viruses enter T-cells through lipid-rich clusters of differentiation (CD-147).^2,6

Mechanism

On host cells, outer spikes of SARS-CoV interact through ACE2 and fuse through TMPRSS2. Such interactions facilitate their entry into the cytoplasm of cells.^1,2,4,5,7 After fusion, ACE-2 present on the surface of cells is downregulated and internalised into the cytoplasm, resulting in its lysosomal degradation and lower ACE-2 expression levels. This reduces the capacity to degrade angiotensin II into I, leading to the accumulation of angiotensin II within the cell, which then binds to angiotensin receptor (AT1R) and releases pro-inflammatory chemicals.^1,2,4,5

How do pro-inflammatory markers cause lymphopenia?

ILs and IFNs act through ILRs and IFNRs, which are present on lymphocytes, leading to activation of the following pathways:^1,8,9,10,11

• NFkB – nuclear factor kappa B
• Janus kinases signal transducer- JAK-STAT
• Cyclooxygenase – COX-2

This will cause cytotoxicity and more cellular insults, recruiting more helper T-cells – T_H cells, macrophages, dendritic cells (DCs) and initiating the vicious cycle of inflammation.^1,8,9,10,11

Early triggers of molecular pathways or inconsistent activation of pro-inflammatory mechanisms cause acute inflammation, which leads to lymphocytosis. However, continuous and persistent SARS-CoV produces chronic inflammation, which damages the nearby healthy and uninfected T-cells and B-cells, exposing more self-antigens, called bystander activation.¹² Such self-products, along with foreign SARS-CoV antigens,  may trigger autoimmunity and reduce lymphocyte count in blood and injury sites. This process is a lymphocytopenia ^12,13

How does COVID-19 (SARS-CoV) act on lymphocytes?

COVID-19 particles behave like PAMPS and bind to class I major histocompatibility complex (MHC-I) molecules. This SARS-CoV-MHC-I complex will be presented to Cluster differentiation 8- CD-8 cells (T_H cells).^8,9,10,11

These CD8 cells act as cytotoxic cells (CTL) and directly kill them through phagocytosis. These same CD8 cells release inflammatory chemicals like IFN𝞬, tumour necrosis factor (TNF-𝞪), and IL-2.^8,9,10,11

How do viruses infect T-cells through CD-147?

COVID-19 binds to CD-147 (TCRs), which is present on lymphocytes. The whole complex (virus-CD147 cells) undergoes internalisation, and the virus fuses with the endosomal membrane, releasing its genome into the cytoplasm. In this manner, a virus replicates its manner, and infection occurs infinitely.^2,6

What is the role of TLRs and NLRs in lymphocytes?

Viruses, after binding to TLR3 or TLR4 or NLRs, will initiate long transductional pathways causing the translation of inflammatory genes like interferon regulatory factor IRF3, NF-kB, inhibitory kappa B kinases (IKK), IL and activator protein-AP-1. In addition,  antigen-presenting cells (APCs) like neutrophils and basophils are also recruited to the infected area and subjected to the release of chemicals like cytokines, chemokines and defensins for killing the viruses.^8,9,10,11

In this way, a storm of cytokines is produced and leads to bystander activation.^{8,9,10,11,12 ,13}

How does T-cell exhaustion occur?

Persistent inflammation and prolonged viral-antigen exposure reduce the cytotoxic potential of T-cell effector cells. For example, this impairs the production of proinflammatory chemicals like ILs and IFNs. This is called T-cell exhaustion.¹⁴

On molecular levels, higher expression levels of the following immune checkpoints induce more brakes on T-cell overactivation, like:¹⁴

• Cytotoxic T cells antigen-4 (CTLA-4) 
• Lymphocyte activation gene 3- LAG-3

Dysfunctional lymphocytes express higher levels of apoptotic marker like:¹⁴

• Programmed cell death protein-1 (PD1)¹⁴ 

As a result, higher PD-1 levels, as well as non-specific killing of nearby lymphocytes due to cytokine outbursts, result in the deletion of T-cells and lower their count in the bloodstream. This process is called lymphopenia.¹⁴

Prognostic markers in COVID-19: lymphopenia

Biological proteins that show the likelihood of disease patterns are called prognostic markers.

Example:

C-reactive protein (CRP)

Higher CRP indicates inflammation.¹⁵

Lymphopenia

Lower lymphocytes in severe COVID-19 infection

Platelets counts

Thrombocytopenia—lower platelet levels occur due to COVID-19's direct entry into bone marrow cells through ACE-2.¹⁵

Neutrophils to lymphocytes ratio (NLR) & platelets to lymphocytic ratio (PLR)

Both ratios are increased in serum due to low T-cell counts (lymphocytes).¹⁵

Procalcitonin (PCT)

Higher values are due to cytokine storm.¹⁶

FAQs

What is the normal range of lymphocytes?

These normally range from 1000 to 4800 lymphocytes per one μL of human blood.

Do lymphocyte levels fall in the bloodstream in severe COVID-19?

Yes, it occurs in severe COVID-19.

If a person suffers from chronic inflammatory disease (CID) like chronic lymphocytic leukaemia (CLL), then what will the lymphocyte count be?

Lymphocytosis occurs in CLL.

How does COVID-19 affect T-cells?

Acute COVID-19 causes lymphocytosis, and in chronic stages, lymphopenia will occur.

What do higher levels of lymphocytes indicate?

It indicates the ongoing inflammatory processes in the early stages of diseases.

Summary

Whenever an inflammation occurs, levels of lymphocytes in the bloodstream are altered. In the case of COVID-19, lymphocyte count varies from one stage to another depending on the severity of the disease. In the early stage, higher production of lymphocytes occurs, whereas chronic inflammation leads to lower T-cell counts in the blood. The SARS-CoV is a foreign antigen that binds to ACE-2 (PRR) in host cells as well as in immune cells and releases proinflammatory markers. This recruits T-cells and then binds to the cytokine receptors – ILR and IFNR present on them. Also, viruses directly bind to lymphocytes through CD147 and TLRs. Intracellular molecular pathways after PAMPs-PRR produce more inflammatory chemicals. Resultantly, prolonged exposure to viruses and resultant persistent inflammation exhaust T-cells. This upregulates the PD-1 and downregulates IL, which causes apoptosis of T cells. In this way, the lymphocyte count drops in the blood and causes lymphocytopenia.