Introduction
Depending on the types of participating cells, recognition patterns, and
effector mechanisms, immunity is traditionally divided into two
categories, namely, innate and adaptive immunity. The former is mediated
by phagocytes, such as monocytes, macrophages, and neutrophils, as well
as natural killer cells (NK cells), and is characterized by a rapid and
non-specific immune response, whereas the latter is mainly mediated by T
and B lymphocytes and is characterized by a slower, albeit highly
specific, response. It is generally believed
that the immunological memory is
associated exclusively with adaptive immunity, and not innate immunity.
Nevertheless, it has long been debated as to whether innate immunity
also has memory feature, and the recent findings have indeed indicated
that immunological memory is not an exclusive hallmark of adaptive
immunity 1, 2. For example, plants and invertebrates,
which lack adaptive immunity, are protected against secondary
infections, and invertebrates can also exhibit transplant rejection3-6. Macrophages, which are key innate immune cell
types, participate in innate immunity by phagocytizing foreign bodies,
and initiate adaptive immunity by capturing and processing antigens,
which are subsequently presented to lymphocytes, thereby playing a
central role in immune responses 7, 8. Moreover,
macrophages exhibit certain immune memory-related properties that are
traditionally attributed to adaptive T and B cells. However, compared
with the memory of adaptive immunity, macrophage memory remains
comparatively unknown. Herein, we describe two types of macrophage
memory, namely, trained immunity (innate immunological memory) and
endowed immunity (adaptive immune-like memory) (Figure 1), as well as
the essential roles these play in a range of diseases.