Biomedical Engineering Reference
In-Depth Information
for the regulation of the immune response (Tizard
2000
; Janeway et al.
2001
) . T-cells are the pre-
dominant blood lymphocyte subpopulation in
ruminants (Tizard
2000
) . The T-lymphocytes
have been subdivided into two main classes, a b
and g d T-cells, depending on the expression of
antigenic markers on the cell surface and cytokine
production. a b T-cells can be further subdivided
into both T helper (CD4
+
) and T-cytotoxic/sup-
pressor (CD8
+
) lymphocytes.
In response to recognition of antigen-MHC-
II complexes and co-stimulatory molecules on
antigen presenting cell (APC) CD4
+
T-cells are
activated and activated CD4
+
cells secrete cer-
tain cytokines that either facilitate a cell-medi-
ated (Th1) or a humoral (Th2) immune response
(Janeway et al.
2001
; Sordillo and Streicher
2002
). The Th1 immune response is character-
ised by increased secretion of IL-2 and INF-g
which in turn enhance cellular responses against
intracellular pathogens and viruses, whereas
Th2 immune response is characterised by higher
production of IL-4, IL-5 and IL-10 supporting
humoral immunity (Kehri et al.
1999
) . However,
in contrast, CD8
+
cytotoxic cells have the capa-
city to kill specific target cells such as tumour
cells or virus-infected cells in combination with
the MHC-I associated molecules. The CD8
+
suppressor cells may produce different sets of
cytokines such as IL-10 and transforming
growth factor beta (TGF-b) that suppress the
immune response (Janeway et al.
2001
) . The
ruminant immune system contains a large pro-
portion of g d T-lymphocytes, and the number
varies with age and is considerably higher in
young animals than in adults, where it consti-
tutes 5-10% of the total peripheral blood lym-
phocytes (Hein and Mackay
1991
) . The g d
T-cells have a wide range of functions, like
secretion of cytokines like interferon gamma
(IFN-g), and cytotoxic activity in response to
intracellular infections. They play an important
role in the early response to infections, prior to
antigen-specific responses are evident setting a
Th1 immune response (Bluestone et al.
1995
;
Baldwin et al.
2002
; Ismaili et al.
2002
; Pollock
and Welsh
2002
) .
3
Heat Stress and Cell-Mediated
Immunity
Heat stress affects peripheral blood mononuclear
cells (PBMC) of cattle and buffaloes, and the
responses are variable and related to physiologi-
cal state of animals. Lymphocyte proliferation
of buffaloes in vitro and IL-1a and b levels are
affected by heat exposure. The levels of cate-
cholamines had significant (
P
< 0.01) negative
effect on lymphocyte proliferation index (LPI)
which indicated that the higher concentration of
catecholamines exhibits a negative impact on
immunity of heat-exposed cells through IL-1a
and IL-1b (Devaraj and Upadhyay
2007
) . The
effects of heat stress on cell-mediated immunity
of bovines have not been evaluated in depth, and
conflicting results have been reported. Soper
et al. (
1978
) reported that hot weather increases
proliferation of PBMC in Holstein-Friesian cows
in temperate climatic conditions. However,
Elvinger et al. (
1991
) demonstrated that prolifer-
ation of bovine lymphocytes was reduced when
cells were incubated for 60 h at 42 °C after stimu-
lation with PHA, PWM or Con A. Kamwanja
et al. (
1994
) reported that in vitro exposure of
bovine lymphocytes to 45 °C for 3 h decreased
the number of viable cells and reduced respon-
siveness of PBMC to mitogens. PBMC from
dairy cattle experiencing temperature-humidity
index (THI) values >72 exhibit reduced prolifera-
tion in vitro in response to mitogenic stimulation
compared with PBMC from cattle experiencing
THI values <72 (Lacetera et al.
2005
) . Incubation
of cattle PBMC at high temperature (42 °C) for
7 h reduced proliferation compared with incuba-
tion at 38.5 °C (Elvinger et al.
1991
) . The precise
mechanism underlying reduced cellular immune
functions during heat stress in cattle are unclear
particularly regarding role of cytokines (Lacetera
et al.
2005
) . The impaired bovine lymphocyte
functions in hot environments might be due to
reduced cellular immunity which in turn influence
the T helper Th1/Th2 balance in favour of the
secretion of Th2 cytokines affecting lymphocyte
proliferation (Lacetera et al.
2005
) .
