Introduction

Lymphopenia, or lymphocytopenia, is the reduction of the number of lymphocytes in the blood. Lymphocytes are white blood cells that play an important role in the fight against infections and diseases. Therefore, lymphopenia is significantly challenging in clinical settings due to the increased risk of developing infections alongside reduced immune function. The condition can arise from various causes, including viral infections, autoimmune diseases and as a side effect of major therapies such as chemotherapy. To improve the quality of life of a person with lymphopenia, it is important to understand the causes and address the condition appropriately.^1,2

Hematopoietic growth factors play a role in stimulating the production, growth and differentiation of blood cells, including lymphocytes. This led to several studies investigating the potential of hematopoietic growth factors as therapeutic applications. Some of the most studied hematopoietic growth factors include Granulocyte Colony-Stimulating Factor (G-CSF) and Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF), which have shown promise in clinical settings, more specifically in people with certain blood cancers or those undergoing intensive chemotherapy.^3,4

This article will summarise lymphopenia as well as the role of hematopoietic growth factors in the treatment of lymphopenia, including their mechanisms of action and potential challenges. 

What is lymphopenia?

Lymphocytes in the immune system 

Lymphocytes, which include T cells and B cells, play an important role in defence against infections and diseases. They generate an adaptive immune response, characterised by immunological memory, after being exposed to new pathogens. This allows for a stronger, more rapid response upon re-exposure to the same pathogen. Memory B and T cells regulate this process, with T cells facilitating a cytotoxic response (causing cell damage or death) and B cells producing specific antibodies against the foreign pathogen.⁵

The adaptive immune response is an especially important component of the immune system, playing a role in long-term protection. Hematopoietic growth factors support the production, proliferation (increase in numbers) and differentiation of these key immune cells, and therefore play a role in overall immune defence. In the case of lymphopenia, where there is a reduction in the number of lymphocytes, their ability of these cells to initiate an efficient immune response is limited. Therefore, methods to increase lymphocyte levels and enhance overall immune function, through hematopoietic growth factors for example, should lead to better quality of life and overall survival outcomes.^2,3,4,5

Causes of lymphopenia

Lymphopenia can arise from a range of causes. nderstanding these causes directs effective interventions. Some of the causes include:²

• Infections (e.g., viral infections: human immunodeficiency virus)
• Autoimmune diseases (e.g., Systemic lupus erythematosus)
• Cancer
• Cytotoxic therapy (e.g., chemotherapy and/or radiation therapy)
• Medications (e.g., corticosteroid therapy)
• Nutritional deficiency (e.g., zinc deficiency)
• Inherited immunodeficiency disorders (e.g., Wiskott Aldrich syndrome)

Symptoms of lymphopenia

Symptoms associated with lymphopenia may include:⁶

• Frequent and longer-lasting infections
• Swelling of the lymph nodes
• Skin rashes
• Enlargement of the spleen

What are hematopoietic growth factors?

The formation of blood cells in the bone marrow, a process termed hematopoiesis, is regulated by hematopoietic growth factors. Hematopoietic growth factors bind to receptors on precursor cells and trigger signalling cascades that lead to the production, growth, differentiation and survival of blood cells. Growth factors direct blood cells to specific lineages; for example, erythropoietin promotes the production of red blood cells. Types of hematopoietic growth factors include:^3,4

• Granulocyte Colony-Stimulating Factor (G-CSF):⁷ G-CSF is a protein that promotes the growth of granulocytes such as neutrophils, a type of white blood cell involved in the innate immune system. A study investigating G-CSF administration in individuals with breast cancer showed an increase in the number of neutrophils as well as peripheral blood lymphocytes, suggesting enhanced immune function

• Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF):⁸ GM-CSF is involved in stimulating the production of both granulocytes and macrophages. A study also highlighted its ability to indirectly mobilise and promote the proliferation of T cells into peripheral blood, enhancing immune function 

• Erythropoietin (EPO):⁹ EPO is a hematopoietic growth factor that promotes the production of red blood cells, and is typically used to treat anemia. Although it is not normally involved in lymphocyte function, studies have shown that EPO has immunomodulatory effects that can influence lymphocytes. However, its effectiveness in treating lymphopenia requires further research

• Interleukin-7 (IL-7):¹⁰ IL-7 plays a role in the development and maintenance of both T and B cells at the early stage. It binds to receptors present on the immune cells and triggers cell survival, differentiation and growth. Its crucial role led to IL-7 being explored as a potential agent targeting lymphopenia by enhancing T cell function 

Clinical applications of hematopoietic growth factors in lymphopenia

One major risk factor in developing lymphopenia is the use of cytotoxic therapy, a treatment that is intended to kill rapidly proliferating cancer cells but, as a side effect, also targets rapidly proliferating lymphocytes. Therefore, lymphopenia is an area of concern, which led to hematopoietic growth factors being suggested as potential agents to support red and white blood cell production.^2,3,4

Research highlights that G-CSF and GM-CSF leads to improvements in lymphocyte levels and immune function, especially in people undergoing chemotherapy or people with an autoimmune condition, both of which are at high risk of developing infections. Additionally, the role of IL-7 in T cell recovery has also been investigated. Elevations in IL-7 were detected in people with human immunodeficiency virus (HIV) infection or lymphopenia due to high-dose chemotherapy. These IL-7 levels reduced as T cell levels increased, suggesting IL-7 plays a role in regulating T cell levels. Another report showed that IL-7, administered as an intervention in cancer patients, can promote the proliferation of naive T cells, suggesting its ability to enhance immune responses.^7,8,11

Challenges of hematopoietic growth factors in treatments

Although several studies have found hematopoietic growth factors to support the production of blood cells following cancer therapy or autoimmune conditions, side effects have been reported. G-CSF, commonly used to treat febrile neutropenia, has been associated with bone pain, acute leukemia and splenic rupture. In addition, EPO analogues, which can be used for the treatment of anemia in cancer patients, have been associated with poorer cancer outcomes and increased mortality in some cases. These effects highlight the need for individualised treatment plans and careful monitoring. While these treatments can improve blood cell counts and reduce the effects of lymphopenia, their potential for complications requires a systematic approach, where risks and benefits are weighed carefully. Further research into the application of hematopoietic growth factors in lymphopenia is also required.⁴

Summary

Hematopoietic growth factors have arisen as important therapeutic agents for promoting blood cell production in patients with lymphopenia, especially during chronic infections, cancer, and autoimmune conditions. Lymphopenia, which is defined as a reduction in lymphocyte counts, can arise from a range of causes, from infections and medications to cancer and cytotoxic therapies. Symptoms associated with lymphopenia may include longer-lasting infections and enlargement of the spleen. Granulocyte Colony-Stimulating Factor (G-CSF), Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF), Erythropoietin (EPO), and Interleukin-7 (IL-7) are among the hematopoietic growth factors which regulate and control the formation of blood cells in the bone marrow. Although these growth factors promote the differentiation of blood cells to specific lineages, studies have shown they can also influence lymphocyte levels. G-CSF and GM-CSF have been shown to increase lymphocyte levels, while IL-7 is suggested to play a role in restoring T cell levels. However, although hematopoietic growth factors have been shown to support the production of blood cells, especially in cancer patients, several side effects have been reported. Further research into the use of hematopoietic growth factors in the treatment of lymphopenia is required, particularly in the context of chronic infections and cytotoxic therapies.