Leptin protects mice from starvation-induced lymphoid atrophy and increases thymic cellularity in ob/ob mice

Abstract

Thymic atrophy is a prominent feature of malnutrition. Forty-eight hours' starvation of normal mice reduced the total thymocyte count to 13% of that observed in freely fed controls, predominantly because of a diminution in the cortical CD4(+)CD8(+) thymocyte subpopulation. Prevention of the fasting-induced fall in the level of the adipocyte-derived hormone leptin by administering exogenous recombinant leptin protected mice from these starvation-induced thymic changes. The ob/ob mouse, which is unable to produce functional leptin because of a mutation in the obese gene, has impaired cellular immunity together with a marked reduction in the size and cellularity of the thymus. We found that ob/ob mice had a high level of thymocyte apoptosis resulting in a ratio of CD4(+)CD8(+) (cortical) to CD4(-)CD8(-) (precursor) thymocytes that was 4-fold lower than that observed in wild-type mice. Peripheral administration of recombinant leptin to ob/ob mice reduced thymocyte apoptosis and substantially increased both thymic cellularity and the CD4(+)CD8(+)/CD4(-)CD8(-) ratio. In contrast, a comparable weight loss in pair-fed PBS-treated ob/ob mice had no impact on thymocyte number. In vitro, leptin protected thymocytes from dexamethasone-induced apoptosis. These data indicate that reduced circulating leptin concentrations are pivotal in the pathogenesis of starvation-induced lymphoid atrophy.

Figure 1

The effect of leptin administration during starvation on lymphoid (a, b, e, and f) and nonlymphoid tissues (c and d) in C57BL/6 mice. (a) Thymic weight, (b) splenic weight, (c) liver weight, (d) kidney weight, (e) thymocyte subpopulations, and (f) splenic subpopulations. Values represent mean ± SEM. *P < 0.05 vs. ad libitum–fed controls; **P < 0.05, ***P < 0.0001 PBS-treated starved mice vs. both ad libitum–fed controls and leptin-treated starved mice.

Figure 2

The effect of leptin administration during starvation on thymic histology in C57BL/6 mice. (a and b) Ad libitum–fed controls. (c and d) PBS-treated starved mice. (e and f) Leptin-treated starved mice. Sections are stained with H&E and shown at ×25 (a, c, and e) and ×250 (b, d, and f). c, cortex; m, medulla; a, apoptotic thymocytes.

Figure 3

The effect of chronic leptin administration on (a) body weight, (b) liver weight, (c) splenocyte subpopulations, and (d) thymocyte subpopulations in ob/ob mice. Values represent mean ± SEM. *P < 0.05 vs. ad libitum–fed controls. **P < 0.05, ***P < 0.0005, PBS-treated starved mice vs. both ad libitum–fed and pair-fed control mice.

Figure 4

Representative flow-cytometric analyses of thymocytes stained for CD4 and CD8. (a) Wild-type mice, (b) ad libitum–fed ob/ob mice, (c) pair-fed ob/ob mice, and (d) leptin-treated ob/ob mice.

Figure 5

Representative flow-cytometric analyses of thymocytes stained with annexin V and PI. (a) Wild-type mice. (b) Ad libitum–fed ob/ob mice. (c) Leptin-treated ob/ob mice. Shown are live thymocytes (lower-left quadrant), thymocytes in the early stages of apoptosis (lower-right quadrant), thymocytes in the late stages of apoptosis (upper-right quadrant), and dead thymocytes (upper-left quadrant).

Figure 6

Representative flow-cytometric analyses of wild-type C57BL/6 thymocytes stained with annexin V and PI, and cultured in vitro with (a) medium alone (b) dexamethasone alone, and (c) dexamethasone and leptin. Shown are live thymocytes (lower-left quadrant), thymocytes in the early stages of apoptosis (lower-right quadrant), thymocytes in the late stages of apoptosis (upper-right quadrant), and dead thymocytes (upper-left quadrant).