In most cancers, tumor hypoxia downregulates the expression of C‐C motif chemokine 2 (CCL2), and this downregulation has been implicated in monocyte infiltration and tumor progression; however, the molecular mechanism is not yet clear. We compared noncancerous and lung‐adenocarcinoma human samples for hypoxia‐inducible factor 1‐alpha (HIF‐1A), microRNA‐210‐3p (mir‐210‐3p), and CCL2 levels. Mechanistic studies were performed on lung adenocarcinoma cell lines and 3D tumor spheroids to understand the role of hypoxia‐induced miR‐210‐3p in the regulation of CCL2 expression and macrophage polarization. HIF‐1Α stabilization increases miR‐210‐3p levels in lung adenocarcinoma and impairs monocyte infiltration by inhibiting CCL2 expression. Mechanistically, miR‐210‐3p directly binds to the 3′untranslated region (UTR) of CCL2 mRNA and silences it. Suppressing miR‐210‐3p substantially downregulates the effect of hypoxia on CCL2 expression. Monocyte migration is significantly hampered in miR‐210‐3p mimic‐transfected HIF‐1A silenced cancer cells. In contrast, inhibition of miR‐210‐3p in HIF‐1A‐overexpressed cells markedly restored monocyte migration, highlighting a direct link between the miR‐210‐3p level and tumor monocyte burden. Moreover, miR‐210‐3p inhibition in 3D tumor spheroids promotes monocyte recruitment and skewing towards an antitumor M1 phenotype. Anti‐hsa‐miR‐210‐3p‐locked nucleic acid (LNA) delivery in a lung tumor xenograft zebrafish model caused tumor regression, suggesting that miR‐210‐3p could be a promising target for immunomodulatory therapeutic strategies against lung adenocarcinoma.

The Bacille Calmette-Guérin or BCG vaccine, the only vaccine available against Mycobacterium tuberculosis can induce a marked Th1 polarization of T-cells, characterized by the antigen-specific secretion of IFN-γ and enhanced antiviral response. A number of studies have supported the concept of protection by non-specific boosting of immunity by BCG and other microbes. BCG is a well-known example of a trained immunity inducer since it imparts ‘non-specific heterologous’ immunity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus responsible for the recent pandemic. SARS-CoV-2 continues to inflict an unabated surge in morbidity and mortality around the world. There is an urgent need to devise and develop alternate strategies to bolster host immunity against the coronavirus disease of 2019 (COVID-19) and its continuously emerging variants. Several vaccines have been developed recently against COVID-19, but the data on their protective efficacy remains doubtful. Therefore, urgent strategies are required to enhance system immunity to adequately defend against newly emerging infections. The concept of trained immunity may play a cardinal role in protection against COVID-19. The ability of trained immunity-based vaccines is to promote heterologous immune responses beyond their specific antigens, which may notably help in defending against an emergency situation such as COVID-19 when the protective ability of vaccines is suspicious. A growing body of evidence points towards the beneficial non-specific boosting of immune responses by BCG or other microbes, which may protect against COVID-19. Clinical trials are underway to consider the efficacy of BCG vaccination against SARS-CoV-2 on healthcare workers and the elderly population. In this review, we will discuss the role of BCG in eliciting trained immunity and the possible limitations and challenges in controlling COVID-19 and future pandemics.