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Blood, Vol. 96 No. 3 (August 1), 2000:
pp. 1180-1183
BRIEF REPORT
From the Basel Institute for Immunology, Basel, Switzerland.
The spleen plays a major role in immune surveillance, but the impact
that splenectomy exerts on the immune competence of an individual is
not fully resolved. Here we show that neonatal splenectomy in sheep
does not abrogate the development of a large, nonrecirculating pool of
lymphocytes and that it has no effect on the acquisition of a normal
blood lymphocyte profile. Splenectomy did, however, result in a
significant decrease in blood residency time of recirculating lymphocytes and in an enhanced accumulation of recirculating
lymphocytes in lymph nodes. Furthermore, nonrecirculating peripheral
blood lymphocytes were less likely to migrate to the lung, possibly because of saturation of the marginal pool by recirculating
lymphocytes. Although splenectomy has little effect on the development
or distribution of lymphocyte subsets in blood and lymph, it has marked
effects on the rate of recirculation of lymphocytes, which may have
significant implications for peripheral immune surveillance in patients
who undergo splenectomy.
(Blood. 2000;96:1180-1183)
As the single largest lymphoid organ in the body, more
lymphocytes migrate through the spleen each day than through any other tissue.1 Data have clearly demonstrated alterations in the distribution of lymphocyte subsets and the activation status of peripheral blood lymphocytes (PBL) after splenectomy.2,3 In
the clinical setting, splenectomy often leads to increased susceptibility to specific infections and abnormal immune
reponses.4 Although splenectomy has been reported to cause
acute decreases in the concentration of circulating IgM antibody, the
most serious consequence is overwhelming postsplenectomy infection
(OPSI).5,6 Although the risk for postsplenectomy sepsis is
significant in adults, this danger is markedly increased in very young
children and has been reported in clinical studies to be as high as
13.8% in children from birth to 5 years of age.7 The
precise mechanism of this immunosuppression is unclear, but it seems
that the spleen contributes significantly to the acquisition of
reactivity to the polysaccharide antigens of pneumococcus and other
encapsulated bacteria, which are the principal agents of
OPSI.6
To understand more fully the role of the spleen in the mammalian immune
system, investigators have examined acute and chronic effects of
splenectomy on the distribution and numbers of peripheral blood
leukocyte subsets. In rats, splenectomy appears to result in a
selective increase in peripheral blood B cells and in CD8+
T cells,8 which may relate to the recently described
nonrecirculating lymphocyte pool (NRLP) found in the blood and spleen
of sheep but not in other lymphoid organs.9-11 Despite the
fact that this blood-borne pool differs functionally and phenotypically
from the recirculating lymphocyte pool (RLP) found in lymph, its
function remains poorly defined. We have used the well-established
sheep model to examine the effects of neonatal splenectomy on the
development of the RLP and the NRLP and the resultant effects on
lymphocyte recirculation.
Study design
Experimental animals
Lymph collection and labeling
Fluorescent cell tracking.
Efferent lymph cells and 350 mL blood were collected and
labeled with CFSE6 and DiI-DS, respectively (Molecular
Probes, Eugene, OR). For DiI-DS labeling, cells were
harvested by centrifugation at 450g for 7 minutes, washed 3 times with phosphate-buffered saline (PBS), and resuspended at 2 × 108 cells/mL RPMI prewarmed to 37°C. Twelve
micrograms DiI-DS/108 cells was dissolved in 300 µL
dimethyl sulfoxide and diluted with RPMI to an equivalent volume to the
cells. Then cells and DiI-DS were mixed gently and incubated for 30 minutes at 37°C. Cells were washed 3 times in PBS and resuspended
in saline for reinjection into the jugular vein. To determine the
proportions of labeled cells, blood samples were drawn at 15-minute
intervals for 4 hours after injection, and blood and lymph were
analyzed at 24, 48, 72, and 96 hours after injection as
described.9
Radioactive tracking.
PBL and efferent lymph lymphocytes (ELL) were collected,
labeled with 111-Indium Oxine (In111) and Sodium
51-Chromate (CR51)12 and
reinjected intravenously. Animals were killed 8 hours later using T-61
euthanasia solution (Cobra II Autogamma Gamma Spectrometer;
Canberra-Packard, Downer's Grove, IL), and samples of tissues were
collected for weighing and radioactive counting as
described.12
Immunophenotyping
Statistics Phenotypic results are presented as the mean percentage of total lymphocytes reactive with antibodies directed against the lymphocyte subsets described above. To assess differences in the migration of labeled lymphocyte subsets in normal and splenectomized sheep, the percentage of each subset within the labeled population in the blood was determined by dual-color flow cytometry. The mean concentration of each subset as measured in 4 animals was then compared to the overall concentration within the blood and between the labeled ELL and PBL recovered from the blood (repeated measures analysis of variance with Tukey post-test; GraphPad InStat version 3.0 for Windows; GraphPad Software, San Diego, CA). To compare the ability of radiolabeled PBL and ELL to home to various tissues in splenectomized and intact animals, the percentage of total injected radioactivity recovered from each gram of tissue was calculated, and the means of 4 animals were calculated. For each tissue, the value obtained for labeled PBL was compared between splenectomized and intact animals, and the value of ELL was similarly compared (Student t test; GraphPad Software).
Development and recirculation of PBL The distribution of T- and B-cell subsets in splenectomized and normal animals is shown in Table 1. Although there did appear to be a slight increase in the percentage of B cells in the peripheral blood of the splenectomized animals, this varied widely between individuals and was not significant. In contrast to previous studies in rodents,1,8 we were unable to identify significant changes in the blood profile after splenectomy. This could be explained as species differences, or it could be that the extended period of time that elapsed between splenectomy and observation in our model (which examined chronic rather than acute immunologic consequences of splenectomy) allowed physiological compensatory mechanisms to function. In rats, the most severe hematologic alterations are observed immediately after splenectomy, with progressive decreases in peripheral blood cell numbers during the following months.8 Nonetheless, the fact that the numbers of B- and T-cell subsets in the blood were normal in the absence of the spleen, which would normally contain a large number of non-recirculating CD21 ve B cells,
indicated that the size of the nonrecirculating lymphocyte pool
develops and is regulated independently of the spleen. This agreed with
previous data obtained in the mouse indicating that the number and
distribution of the peripheral T- and B-cell pools are regulated at the
pool level rather than through thymic input.15,16
Enhanced migration of the RLP after splenectomy
Conclusion
We thank Drs Alexandre Potocnik and Marco Colonna for critical reading of the manuscript and Dr Jack Hay for helpful discussion.
Submitted November 24, 1999; accepted March 24, 2000.
The Basel Institute for Immunology was founded and is supported by F. Hoffmann LaRoche Ltd.
Reprints: Alan J. Young, c/o Steven Mentzer, Brigham and Women's Hospital, Department of Thoracic Surgery, 75 Francis Street, Boston, MA 02115; e-mail: shepherd666{at}hotmail.com.
The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked "advertisement" in accordance with 18 U.S.C. section 1734.
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