From the Guidelines
Elevated monocytes, or monocytosis, can be caused by various infections, including COVID-19, which has been shown to provoke a systemic cytokine release and a resultant cytokine storm in select patients with severe disease, leading to monocyte and macrophage hyperactivation 1.
Infections Causing Monocytosis
- Bacterial infections: tuberculosis, subacute bacterial endocarditis, brucellosis, and typhoid fever
- Viral infections: Epstein-Barr virus (infectious mononucleosis), cytomegalovirus, HIV, and COVID-19 1
- Parasitic infections: malaria, leishmaniasis, and trypanosomiasis
- Fungal infections: histoplasmosis and coccidioidomycosis
- Rickettsial diseases: Rocky Mountain spotted fever
Mechanism of Monocytosis
Monocytes are key components of the innate immune response, functioning as phagocytes that engulf pathogens and present antigens to activate the adaptive immune system. They also produce cytokines that regulate inflammation. In the context of COVID-19, monocyte and macrophage hyperactivation may contribute to increased circulating levels of pro-inflammatory cytokines, including tumor necrosis factor (TNF)-α, interleukin (IL)-6, IL-1, and interferon (IFN)-γ 1.
Clinical Implications
Understanding the causes of monocytosis is crucial for diagnosing and managing underlying infections. In the case of COVID-19, recognizing the role of monocytes in the immune response can inform therapeutic considerations, particularly in patients with overlapping COVID-19 and cardiovascular disease 1.
From the Research
Infections Causing Elevated Monocytes (Monocytosis)
- Tuberculosis (TB) is a major infection that can cause elevated monocytes, as seen in studies 2, 3
- Monocytes play a crucial role in the primary innate immune response to TB, and alterations in monocyte subsets have been observed in TB patients 2
- The frequency of intermediate and non-classical monocyte subsets is altered in TB, suggesting their impact on bacterial persistence 2
- Monocyte-related gene biomarkers have been identified to distinguish between latent and active TB, highlighting the importance of monocytes in TB pathogenesis 3
- Other infections, such as HIV, can also affect monocyte counts, and the interaction between HIV and TB can impact monocyte subsets and function 4, 5, 6