The second major area of immunological development has been
the realisation that regulation of the adaptive immune response
is undertaken by numerous different T cell subsets. The recognition
that T lymphocytes include CD4+ T helper (Th) cells, CD8+
T cytotoxic (Tc) cells and T “suppressor” cells dates from several
decades ago. The 1986 description of functional subsets of CD4+
T cells in the mouse has led to an explosion of knowledge in this
area, which has fundamentally reshaped our understanding of
the immune system and has affected the diagnosis and management
of immune-mediated disease and the development of novel
therapies and vaccines (Mossman and others 1986). Of greatest
importance was the initial recognition of Th1 and Th2 subsets that developed from a common Th0 precursor cell dependent on
the signals delivered to that precursor by the APC (as described
above).
Th1 cells are characterised by production of the “signature
cytokine” interferon (IFN)- γ and their effect is in inducing cellmediated
immunity (CMI) involving processes such as target cell
cytotoxicity (e.g. of tumour cells or virally infected cells by Tc)
or macrophage killing of intracellular pathogens (e.g. Leishmania
and Mycobacterium). In contrast, Th2 cells produce a panel
of cytokines including interleukin (IL)-4, IL-5, IL-9 and IL-13,
which permit them to activate B cells allowing transformation to
plasma cells and secretion of antibody [e.g. immunoglobulin (Ig)
G, IgA or IgE] in the humoral immune response. Other “effector
cell” populations have more recently been characterised in experimental
rodents and man, including Th17 cells (producing IL-17)
that play a role in the immune response to infectious agents (Bettelli
and others 2007) and Th9 cells (producing IL-9) that may be
important in the allergic immune response (Soroosh and Taylor
2009). There is mounting evidence that dogs and cats also harbour
these T cell subsets and particular immune responses and
immune-mediated diseases in these species are characterised by
cytokine profiles consistent with either Th1/Th17 or Th2/Th9
immunity (Horiuchi and others 2007, Saridomichelakis 2009).
Although the above Th subsets all promote different arms of
the immune response, there are other CD4+ T cell subsets that
inhibit or suppress immune responses. These are now termed
“regulatory” T cells (Treg) but are in effect the same populations
as the originally described T “suppressor” cells. A number of different
types of Treg are now described including Th3 cells (that
produce the cytokine-transforming growth factor β) and induced
Treg that are stimulated in the context of specific immune
responses. Most important of all are the “natural” Treg that
are continuously present within the immune system and have
the effect of preventing the activation of potentially damaging autoreactive or allergen-reactive T cells (O’Garra and Vieira
2004). These cells are characterised by production of the key
immunoregulatory cytokine IL-10 and by expression of a range
of other markers (species dependent) including the IL-2 receptor
(CD25) and Foxp3. Treg cells have now been identified in both
dogs and cats (Biller and others 2007). In man and dogs, it is now
clear that one part of the immunology of allergic and autoimmune
disease is under-function of these regulatory populations.
The functional subsets of CD4+ T cells are summarised