Immunology

1. Primary Lymphoid Organs—Where Immune Cells Develop
• The self-renewing and differentiation ability of any
stem cell depends on the structural organization and
cellular function of stem cell niches (specialized
anatomic microenvironments)
• These sequestered (segregated or isolated) regions are
typically populated by a supportive network of
stromal cells
• Stromal cells express soluble and membrane-bound
proteins that regulate cell survival, proliferation,
differentiation, and trafficking.

2. • Over the course of embryonic development
there is actually a change in the organs that
have microenvironments that support the
differentiation of hematopoietic stem cells
• However, by mid to late gestation (the process
or period of developing inside the womb), HSCs
take up residence in the bone marrow, which
remains the primary site of hematopoiesis
throughout adult life.

3. • The bone marrow supports
– the maturation of all erythroid and myeloid cells
and,
– the maturation of B lymphocytes (in humans and
mice).
• T lymphocytes do not complete their
maturation in the bone marrow.
– their precursors need to leave the bone marrow
and
– travel to the unique microenvironments provided
by the other primary lymphoid organ, the thymus,
in order to develop into functional cells.

4. • HSCs are also found in blood and may naturally
recirculate between the bone marrow and
other tissues.
• This observation has simplified the process
used to transplant blood cell progenitors from
donors into patients who are deficient (e.g.,
patients who have undergone chemotherapy).
• Whereas once it was always necessary to
aspirate bone marrow from the donor—a
painful process that requires anesthesia—it is
now sometimes possible to use enriched
hematopoietic precursors from donor blood,
which is much more easily obtained.

5. • A primary lymphoid organ that supports self-
renewal and differentiation of hematopoietic stem
cells (HSCs) into mature blood cells.
• Although all bones contain marrow, the long bones
(femur, humerus), hip bones (ileum), and sternum
tend to be the most active sites of hematopoiesis.
• The Bone Marrow is responsible for
– the development and replenishment of blood cells
– maintaining the pool of HSCs throughout the life of an
adult vertebrate.
The Bone Marrow

6. a typical cross-
section of a bone
with a medullary
(marrow) cavity
The Bone Marrow
Multiple bones support hematopoiesis,
including the hip (ileum), femur, sternum, and
humerus.

7. • The adult bone marrow, the exemplary adult stem
cell niche, contains several cell types that
coordinate HSC development, including
(1) osteoblasts, versatile cells that both generate bone
and control the differentiation of HSCs,
(2) endothelial cells that line the blood vessels and also
regulate HSC differentiation,
(3) reticular cells that send processes connecting cells to
bone and blood vessels, and
(4) sympathetic neurons, which can control the release
of hematopoietic cells from the bone marrow.

8. Microscopic cross-section reveals that the bone marrow is
• tightly packed with stromal cells and hematopoietic cells
at every stage of differentiation.
• With age, however, fat cells gradually replace 50% or
more of the bone marrow compartment, and the
efficiency of hematopoiesis decreases.
a typical cross-section of a bone with a
medullary (marrow) cavity.

9. • Various niches in bone marrow play different
roles:
– the endosteal niche (the area directly surrounding
the bone and in contact with bone-producing
osteoblasts) and
– the vascular niche (the area directly surrounding
the blood vessels and in contact with endothelial
cells)

11. Blood vessels (central sinus and medullary artery) run through the center of the bone and
form a network of capillaries in close association with bone and bone surface
(endosteum). Both the cells that line the blood vessels (endothelium) and the cells that
line the bone (osteoblasts) generate niches that support hematopoietic stem cell (HSC)
self-renewal and differentiation. The most immature cells appear to be associated with
the endosteal (bone) niche; as they mature, they migrate toward the vascular (blood
vessel) niche. Fully differentiated cells exit the marrow via blood vessels.

12. It is important to recognize that the bone marrow is
not only a site for lymphoid and myeloid
development but is also a site to which fully mature
myeloid and lymphoid cells can return.
Mature antibody-secreting B cells (plasma cells) may
even take up long-term residence in the bone
marrow.
Whole bone marrow transplants, therefore, do not
simply include stem cells but also include mature,
functional cells that can both help and hurt the
transplant effort.

13. The Thymus
• T cell development is not complete until the cells
undergo selection in the thymus
• The importance of the thymus in T-cell development was
recognized during the early 1960s.
• popular assumptions for thymus were
– a graveyard for cells.
– an underappreciated organ, very large in prepubescent
animals, that was
– thought by some to be detrimental to an organism,
– others thought it to be an evolutionary dead-end.
– The cells that populated it—small, thin-rimmed, featureless
cells called thymocytes—looked dull and inactive.
• J.F.A.P. Miller, an Australian biologist, proved that the
thymus was the all important site for the maturation of
T lymphocytes.

14. Thymus
Bone marrow
Cortex Medulla
The thymus is found just above the
heart and is largest prior to puberty,
when it begins to shrink. a stained thymus tissue section

15. • T-cell precursors, which still retain the ability to
give rise to multiple hematopoietic cell types, travel
via the blood from the bone marrow to the thymus.
• Immature T cells, known as thymocytes (thymus
cells) because of their site of maturation, pass
through defined developmental stages in specific
thymic microenvironments as they mature into
functional T cells.
• The thymus is a specialized environment where
immature T cells generate unique antigen
receptors (T cell receptors, or TCRs)

16. • Immature T cells are then selected on the basis of
their reactivity to self MHC-peptide complexes
expressed on the surface of thymic stromal cells.
• Those thymocytes whose T-cell receptors bind self
MHC-peptide complexes with too high affinity are
induced to die (negative selection)
• Those thymocytes that bind self MHC-peptides
with an intermediate affinity undergo positive
selection, resulting in their survival, maturation,
and migration to the thymic medulla.

17. • Most thymocytes do not navigate the journey through
the thymus successfully; in fact, it is estimated that 95%
of thymocytes die in transit.
• The majority of cells die because they have too low an
affinity for the self-antigen-MHC combinations that
they encounter on the surface of thymic epithelial cells
and fail to undergo positive selection.
• These developmental events take place in several
distinct thymic microenvironments.
• T-cell precursors enter the thymus in blood vessels at the
corticomedullary junction between the thymic cortex,
the outer portion of the organ, and the thymic medulla,
the inner portion of the organ

18. a cartoon of the microenvironments: the cortex, which is densely populated with
DP immature thymocytes (blue) and the medulla, which is sparsely populated with
SP mature thymocytes. These major regions are separated by the
corticomedullary junction (CMJ), where cells enter from and exit to the
bloodstream. The area between the cortex and the thymic capsule, the
subcapsular cortex, is a site of much proliferation of the youngest (DN)
thymocytes. The route taken by a typical thymocyte during its development from
the DN to DP to SP stages is shown. Thymocytes are positively selected in the
cortex. Autoreactive thymocytes are negatively selected in the medulla; some may
also be negatively selected in the cortex

19. • At this stage thymocytes express neither CD4 nor
CD8, markers associated with mature T cells.
They are therefore called double negative (DN)
cells.
• DN cells first travel to the region under the
thymic capsule, a region referred to as the
subcapsular cortex, where they proliferate and
begin to generate their T-cell receptors.
• Thymocytes that successfully express TCRs begin
to express both CD4 and CD8, becoming double
positive (DP) cells, and populate the cortex, the
site where most (85% or more) immature T cells
are found.

20. • The cortex features a distinct set of stromal cells,
cortical thymic epithelial cells (cTECs), whose
long processes are perused by thymocytes testing
the ability of their T-cell receptors to bind MHC-
peptide complexes.
• Thymocytes that survive selection move to the
thymic medulla, where positively selected
thymocytes encounter specialized stromal cells,
medullary thymic epithelial cells (mTECs).
• Not only do mTECs support the final steps of
thymocyte maturation, but they also have a
unique ability to express proteins that are
otherwise found exclusively in other organs.

21. a cartoon of the microenvironments: the cortex, which is densely populated with DP immature thymocytes
(blue) and the medulla, which is sparsely populated with SP mature thymocytes. These major regions are
separated by the corticomedullary junction (CMJ), where cells enter from and exit to the bloodstream. The
area between the cortex and the thymic capsule, the subcapsular cortex, is a site of much proliferation of
the youngest (DN) thymocytes. The route taken by a typical thymocyte during its development from the
DN to DP to SP stages is shown. Thymocytes are positively selected in the cortex. Autoreactive thymocytes
are negatively selected in the medulla; some may also be negatively selected in the cortex

22. • This allows them to negatively select a group
of potentially very damaging, autoreactive T
cells that could not be deleted in the cortex
• (Note that some investigators describe
positive selection as taking place in the cortex
and negative selection solely in the medulla.
However, several lines of evidence suggest
that negative selection can also occur in the
cortex)

23. • Mature thymocytes, which express only CD4
or CD8 and are referred to as single positive
(SP), leave the thymus as they entered: via the
blood vessels of the corticomedullary
junction.
• Maturation is finalized in the periphery, where
these new T cells (recent thymic emigrants)
explore antigens presented in secondary
lymphoid tissue, including spleen and lymph
nodes.